Niagara,
I'm not sure I can help, but I CAN tell you that you aren't alone. I, too, have experienced problems with seeing open wounds, i.e., feeling lightheaded like I was going to faint--actually DID faint on two occasions before I was in medical school.
I think it is really important to recognize patterns of "triggers." For me, I seem to have the most difficulty when I KNOW the patient is in pain. Right now I am on my surgery rotation but don't seem to have any problems because I know the patient is knocked out under general anesthesia. Other situations that were messy--such as autopsies, anatomy lab dissection--never bothered me. I will probably have more problems with something like ER where the patient is in obvious distress.
Here are a few things I've learned both firsthand and from other students:
--always make sure you eat before surgery
--drink enough water that you aren't dehydrated, but keep in mind you don't want to be peeing every 5 minutes either (not very compatible with a surgery rotation).
--focus on the anatomy and the procedure. I think it helps if you actually get to DO something in the case rather than just watch; keeps your mind occupied.
--don't lock your knees!
--shift your weight around throughout the case, i.e., put most of your weight on your left leg for awhile, then change to your right leg. You don't want to stay in one position too long.
--try occasionally tightening up some large muscle groups and then relaxing. Quads are easy to do. This helps keep the blood moving around in your legs instead of just pooling.
--warning signs that you may be in for problems:
1. feeling hot; clammy sweat. This is my first warning sign.
2. feeling lightheaded, woozy. This usually comes next. If I get to this stage, I start thinking about what to do if I keep progressing to #3.
3. "buzzing" or "ringing" in the ears.
4. losing peripheral vision (starts blacking out); get tunnel-like vision. You will definitely go DOWN if you don't do something NOW.
5. sometimes other people will tell you that you look pale.
--in the unfortunate case you DO start to feel any of the above, especially to the point where you are lightheaded, take precautions IMMEDIATELY. DO NOT TRY TO "TOUGH IT OUT!" It is better to feel like a wuss for a few seconds while you sit down than to pass out in the sterile field, pass out and cut your head when you hit the ground, etc.
1. If you are merely observing the surgery, sitting down on a stool (if you can find one nearby) may fix the situation. Otherwise, back up to a wall (watch out for anything sterile) and slowly slide down into a seated position using the wall for support. If you try to leave the room, you may not make it! I tried leaving the room once in the ER. As soon as I was out the door, I collapsed in the hallway on my face and wound up with a bloody lip and nose. Not cool. VERY embarrassing!
2. If you are scrubbed in, let someone know you are feeling ill so they can take over your job, then get the hell out of the way and try #1.
3. Put your head down while you're seated. It will feed blood to your poor hypoxic brain and help clear the lightheadedness.
4. Get cooler. All that surgery gear (gown, lead apron, etc.) can get hot anyway even if you're NOT feeling faint. A cool, wet towel helps, too.
BOTTOM LINE: Better to feel foolish and PREVENT syncope than deal with the consequences of crashes.
I didn't know this until I took psych, but there is actually a name for this condition: blood-injection-injury phobia. I've never felt "scared" of blood or anything like that, and I've certainly never been "diagnosed" or anything, but I wouldn't be surprised if I met the criteria (whatever those criteria are). BII phobia is different than other phobias because it is a parasympathetic response--a vasovagal reaction. Heart rate slows down instead of taching; therefore less blood gets to the brain and you feel faint. If it makes you feel any better, exposure is supposed to help "cure" it.
I don't know if I have helped you at all. Hope so anyway.
Feel free to PM me if you wish.
Good luck!
UnderGrad
Ironically, I was perfectly fine in anatomy lab. 
