Having been surfing for some time now I've also suffered nasty cuts and a few serious problems that have kept me out of the water for extended periods of time. Here's a few tips that can help you avoid inury and increase your time in the water.
Always surf with other people. I've known very good surfers who've had to be saved because of "freak accidents." You may get tired, get an unexpected cramp, be caught in a strong current, even suffer a dislocated joint that prevents you from getting back to shore. If nothing else, think of them as shark buffers :) When I took a 5 inch gash that opened up my thigh I was so happy that I had begged my friend to come along.
Know your abilities. If you're a beginner, stay where the beginners are -inshore and away from the experienced surfers and best surfing areas. Don't try to paddle out into the lineup when the waves are too big.
Always watch the waves, watch your board, and watch for other people. How many people have I met who've told me about getting slammed by a wave the second they turned around? Keep your board at your side and not infront of you. And when a surfer is surfing towards you move yourself in the opposite direction in which he is headed (go toward the already broken part of the wave). If he's coming straight for you and you really can't move, dive deep under the water and use your board as a shield. Your boards can be replaced. Your body can't.
Tuck 'n Cover. When you wipe out, curl into a fetal position and cover your face and head with your arms and hands. Really simple and intuitive motion. This is especially important when you're being tossed around under water by a heavy wave and when you don't know where your board is. Try to relax as you're being held down because struggling will only make you lose your air and then you'll drown. Remember that though it feels like an eternity, you'll actually only be down for 15-20 seconds in the heaviest of waves. When the force lets up on you, come up slowly with your arms and hands still covering your face & head. Look for your board and then look for the next wave coming down ontop of you :)
Big Waves: Don't try to go through them with your longboard (or any board for that matter). So what do you do? Make sure there's no one near you, ditch your board pushing it away from you but towards the wave, and then dive under and through the back of the wave. You'll most likely get pulled back (and probably under) by the board being caught in the wave so be prepared to tuck 'n cover as well as hold your breath. **Important** if people are nearby, do not let go of your board. Turn turtle/barrell roll and hold on very very tight. Keep your arms stiff and don't hit your head on the board.
No backing out. When you've made the decision to drop into a wave, do it with everything you've got. Do not try to back out of a steep wave or else you risk "going over the falls" and getting hurt bad. If you are going to pearl (where the nose of the board dives straight into the water), quickly roll off the side of your board into the water, and tuck 'n cover.
Give your back a break. Arching allows you more leverage and thrust when paddling but too much can cause muscle overuse injuries. Avoid arching for extended periods of time. Sit up on your board when you're waiting around. Bend your knees so that your feet are in the air when you're paddling. When paddling long distances on fairly calm water (usually past the lineup), put your head down so your back straightens out.
Warm up & Stretch. Cold muscles & joints are prone to injury. Do some light warm ups and stretch your muscles. Also take the time to warm up your rotator cuffs which are prone to injury and overuse and exercise them when you are not surfing. See this page for rotator cuff exercises.
Respect the Ocean. You are just a speck floating in it. It can toss & tumble you like nothing. Whether you are paddling out or being put through the washing machine, the best thing to do is relax and wait for it to calm down. Always keep an eye out for the wave that's going to come down on your head.
Know when enough is enough. You may be good but when you're tired you won't have the strength to get out of tough spots. Call it a day and head in for some food :)
