You want to seed yourself appropriately. If you know you're in the top 25% for finishing times then try to seed yourself with about 25% of the people in front of you. If you're a good strong swimmer that sights well, being on the side of the pack closest to the bouys is a good option. If you warm up slowly into your swim, you'll want to try to find a spot with few people behind you. If you're a weak swimmer, staying to the back and to the outside of the group is a good thing to do and if you're really worried about getting pushed around a bit, there is no reason why you can't wait a few seconds while everyone else in the wave sets off while you start 10 to 15 seconds later. You don't lose much time and you might gain a lot in terms of energy conservation by not being stressed out by the other swimmers.

The larger the waves, the more likely you'll get pushed around a bit. A 20 to 30 person wave spreads out quickly and is usually quite civil.