Lesson 1: Push More (You Don't Have Pull out of Every Corner)
Rookie rowers tend to pull away from every obstacle. They know that pulling is their strongest stroke, and they are beginners, so any rock or bend in the river can trigger them into "backwards" mode. They set up way ahead of time and start pulling and pulling, and really work up a sweat by the bottom of the rapid. This is by and large unnecessary. If they simply dropped their oars and picked up a beer, they'd probably float right around the dreaded obstacle they worked so hard to avoid. This is not to say that you should never pull, but at least play with pushing more.
I had the good luck one time to hike in a Phantom and step onto a raft rowed by one of the best boatmen of our time. The flow was around 25,000 cfs (probably more due to monsoon rains), and he pushed the boat through Horn, Granite, Hermit, Crystal and all the Gems, with nary a backstroke to remember. He joked about how they worked to never ever pull, always push, all the way down the river.
Taking a page out of his book I started to experiment with pushing. Obviously you can read water better because you are looking downstream. I discovered that you can push around most corners (in the Grand Canyon at least) because the laterals coming off the bank keep you from hitting it. Instead of freaking out that you will splat on shore, you push toward the inside of the turn, putting your boat broadside to any small or medium laterals coming off the obstacle you'd like to avoid. If you nudge the boat in the right direction, the laterals will usually surf your away from the obstacle. You shouldn't be working super hard when you are pushing; the advantage comes from being able to see and using angles and the water to put you where you want to go.
Pushing through rapids with moves does require that you stay on the job every moment. To run rapids you choose a line that you think you can make, and start every move early. You can't make last minute moves, because pushing is a weaker stroke. You can't relax at the bottom of the rapids because you'll get spun into an eddy and spend 1000 calories getting back out. You are on duty as long as you are in the driver's seat.
Pushing makes your whole trip move downstream better. If every rower in your group pushes through most rapids, you will spend fewer afternoons battling headwinds. Groups with a mix of experienced rowers who push, and rookie rowers who pull, have two kinds of trouble. If a puller is in front of a pusher, there will be crowding and passing if not collisions in rapids. And if the pushers get in front, the group can get split up by a mile. For the sake of trip flow, see if you can get with the pushers and make time for lunch hikes and early camps.
As with every technique, there are times when pushing does not work. If you have a massive amount of momentum toward shore, you might need to pull a couple times to neutralize that momentum. Turning your boat sidesways to laterals does not work for avoiding undercuts because they do not generate laterals. Turning your boat sideways to a lateral that is so big it might flip you, like the one at the top left of Upset rapid, can also be counterproductive. Practice steadily pushing through easy and moderate rapids and save pulling for when you really need it. Your rowing days will get shorter and easier.
Lesson 2: Ditch the Training Wheels
When people first start rowing, they often have a setup that keeps the oars from twisting. This can be accomplished using oar rights with locks or pins and clips. Both of these systems allow the rower to drop the oar in the water, and grab it again knowing that pushing the handle down will bring the blade slicing back to the surface. This is very reassuring and helpful to the rookie rower, who is still figuring out how to push, pull, pivot, and manage the considerable momentum of a large raft. As long as your hands are full just hitting the lines, it's not a bad idea to use rites or pins to keep the task as simple as possible.
Once you're hitting most of your lines and feeling confident, you graduate from novice and become an advanced beginner. One way to step it up is to row with open oarlocks, at least some of the time. If you were using rites with stoppers, it's easy to take them off at any point on a rowing trip, and put them back on when you want. All it takes is a screw driver. If you're using pins and clips, you'll probably have to wait for the next trip to try open locks.
The advantage of rowing with open oarlocks is that you can feather the blades of your oars through the water and the air. You naturally twist the oar a little bit with each stroke, so you learn how to bring the oar back to the correct angle with every stroke. When the oar gets stuck under the water, you have to twist the handle until the blade begins slicing the way you want it to. Dory boatmen have been known to use their oars to brace and prevent a flip, but in a rubber raft I don't know of this ever working. Once you get the feel of rowing with open oarlocks, it is the most natural and comfortable thing in the world. Until that point it's just a challenge to keep from whiffing strokes or getting your oars taken away by the river. Try it on a run or section that is easy for you, so you have a little time to get your brain rewired for managing blade angle on top of everything else.
On the down side, you need a stronger grip because you have to hold onto the oars a little harder to keep control of them. If you're not strong to start with you can easily develop tendonitis in the wrists or elbows, so moderation is advised. Grip and forearm exercises before the trip can help a lot. The transition from fixed-oar rowing to open oarlock rowing can be as fast or slow as you want it to be, but I do suggest that you give it a shot and learn the joys of feathering your blades. You might like it.
Lesson 3: Ergonomics of the Boat
If you just can't row for shit, the boat probably doesn't fit you. An excellent boatman trying to row a bad setup might as well be tied up. When you get the oars and seat and foot braces in the right place for you, your body will be in its strongest position. Then you have a fighting chance. Not every part of a row rig is adjustable, so you will suffer some in the learning curve. Get the right length oars relative to the boat, because you can't adjust oars at the put-in. You also can't adjust the dimensions of a welded frame, so hopefully the one you end up with is not weirdly sized.
Most parts of a row rig are adjustable. Ask a veteran and get some help adjusting your boat to fit you. If you are going on a longer trip, you end up tinkering with your boat many times as you learn the hard way how each adjustment affects your rowing.
Most folks row with the oar stands at about the midpoint of the boat's length. Some prefer them to be toward the rear so that they can rudder more and use paddle power from passengers in the front to move forward. These are known as center mount or rear mount rigs and require different rowing styles.
Some of the possible adjustments include oar stand location and angle, seat height, foot brace locations, and fulcrum location on the oars. On many frames you can slide the oar holders up and down the frame side tube just by loosening a couple of set screws. Adjust the forward-back location of the oar holders to get the middle of your stroke to occur when the blade is at 90 degrees to the boat. When you find the right spot, tighten the set screws correctly, tight enough but not damaging the frame. Get help with this.
You have to be able to reach the water with your oars, and to easily lift your blades out of the water. If you are particularly tall or short, you may need a special length oar stand; investigate your options well ahead of time. Adjust your seat height and the tilt of your oar stands until your push stroke occurs at shoulder height with the blade fully immersed.
Depending on your height you may wish to strap a pair of ammo cans in the cockpit for an intermediate footrest. For heroic hard pulls, most boatmen put their feet on the frame in front of them because this yields the most power per stroke. For general rowing this stance is not comfortable, and a midpoint foot rest is nice to have. For pushing, most people sit toward the front of their seat with their tippy toes touching the floor. If you can't reach the floor, you might want to put something under your feet for comfort there too. There is an opposing view that says keep your floor clear as possible to make it easier to hunker down in there without getting dinged on hard boxes. The choice is ultimately up to each boatman, how many and what kind of footrests you'd like to rig.
Last but not least, your personal strength and power should determine where you put the fulcrum of your oars. The location of the oar stopper or clip on the oar determines what gear you will be in as you row down the river. Guys often rig their oars so that when they stretch their arms out with hands on the oar grips, their hands are shoulder distance apart. This arrangement happens because the stopper/clip is close to the oar handle. I call this macho man rowing, because it puts you in a high gear, and you have to be very strong to take a stroke. The advantage is that if you are strong enough to manage it, you can really move the boat fast.
If you are puny like me and prefer granny gear, you can set your oars up "girlie style". I put my stoppers so far out that when my arms are outstretched in front of me on the oar grips, my hands overlap. This puts me in a nice low gear which allows me to take strokes without the oars getting ripped out of my hands so often. There is a hazard involved, however, and that is the pinched thumbs that you get if you wrap your thumbs around the ends of the oars and forget to cross them. After a few pinches you will learn to cross them without having to think about it.
Of course there are inches of middle ground between macho and girlie: you get to play with it and figure out what the right fulcrum is for you. Stoppers tend to work their way toward the handle with rowing, so you may start out girlie style and end up macho style without knowing it. This is fine but you should check your oars daily to make sure it hasn't "self adjusted" to some arrangement that doesn't work for you.
It's also wise to check your rowing apparatus daily. I've had pins pop out, and oarlocks too, right in the middle of some pretty big rapids, so make sure those babies and the oar stands are secure. I've seen oar rights get twisted on the oar shaft, and oar stands get tweaked out to the side, and coolers get shimmeyed right out of their straps. Develop a routine that involves a 20 point "flight check" to make sure that your boat doesn't come apart on you when you really wish it would hold.
Bottom line is that when you first start out you aren't likely to have a clue how to make the boat fit you, but you will learn. Ask questions and get help, instead of just suffering. Most likely there's a way to personalize any borrowed or rented rig so that it fits you, at least better than it did when you got it. Get that allen wrench out of the repair kit, and start tinkering.