First...calm down, if you can be prepared mentally for the open water swim, you'll be fine. I feel like I just get pushed along with all the energy at the start of the race!

It's a great idea to practice in the open water as much as possible, in the body of water where the race is if possible. Also, practice sighting - look ahead at a fixed point (the bouy during competition) every 15 strokes or so to swim straight. Also, if you can practice with a big group, or with a friend swimming REALLY close to you, that will get you used to the crowded swim course.

The swim usually flies by really fast for me because I get so excited, but just make sure you can do the distance without stopping, and have a back up stroke (mine is the breast stroke) besides freestyle, and be prepared to use it, to calm down and breathe during the race.

Good luck!