I did a triathlon a couple years ago. I have always been a pretty strong cyclist so that part was not very difficult for me, I really struggled with the swim. I only practiced indoors and thought I was set cause I could hammer out fast laps all day long. Well, open water swimming is quite different that in a pool...especially when there is hundreds of people swimming inches from you. There is no line to follow so you need to be able to keep yourself straight by looking up between strokes.
So if I were to give you one piece of advice I would say practice in the open water. But if you live near where it will be go ride and run the course a couple times so there is no surprises in the terrain. Have fun and good luck.