I usually just follow the instructions on the zipper packaging as a guideline. I used this tutorial (
http://neveryawn.wordpress.com/2007/05/27/tutorial-pillow-cover-with-invisible-zipper/) for inserting an invisible zipper for a pillow. This should be similar to your dress, since the zipper is installed before the dress is assembled.
The key to invisible zippers is that their coils kind of roll over a bit, so you want to hold it opened/unrolled as you sew in order to get your needle to sew as close to the coils as possible. It's also a good idea to check how close you got after sewing one seam and sewing again if necessary before moving on to the second piece.
There are invisible zipper feet, but I just use my regular zipper foot.