Eat breakfast, snacks, lunch, more snacks... the worst mistake students make is thinking "If I eat, I will surely throw it back up..." Instead, you end up passed out cold on the floor from low blood sugar.
Never lock your knees! As someone else said, shift your weight. If you start getting that weak, jello-legged feeling... get out of the room/area and sit. Drop to the floor against the wall and sit if you have to. Find something and fan yourself cool. Go find some grape juice or a piece of hard candy.
If you're in the OR, don't get too close to the sterile field. Trust me, as someone who was a former scrub tech, it is NOT fun to have a student pass out on your sterile field! You will do much better to be honest and excuse yourself from the room before you contaminate everything and make a lot of people very unhappy.
If you notice you're starting to get grossed out, immediately look away and change your thoughts as you remove yourself from the area. You may spare yourself some embarrassment later.
Always have an escape plan. Even if you're not normally claustrophobic by nature, you stick yourself in an uncomfortable, gross situation and you may find yourself feeling trapped, and panicky. Have a way to escape and excuse yourself gracefully.
It's not always the obvious gross stuff that will get you. I nearly passed out during my NICU rotation from watching an umbilical line being pulled. It wasn't bloody, it wasn't painful for the baby but it sure made the color drain from my face!!! Everyone has their limits. Know yours.
Lastly, NEVER make yourself feel the pain of the other person. IF you try and empathize to this point, you're going to feel the effects. If you're in there changing that dressing or cleaning out that wound and you're attempting to transfer their pain and discomfort upon yourself, you WILL gross yourself out. Don't do it!