I finished Whitney's skirt zipper yesterday, and I'm so excited to tell you that I learned a technique doing invisible zippers over a seam line (which I have had trouble before).
The picture above is the inside view of an invisible zipper already sewn in. The first thing I did was sew up one side the invisible zipper, keeping the seam allowances OPEN where the zipper hits. This will make it less bulky and easier to zip up when on (and i think this may be the problem I had before-- I was sewing the zipper in with the seam allowances together, facing down. Not good). Now, a lot of you may know, if you have sewn in an IZ (invisible zipper) before, is that it is a beast trying to get the seam lines to match perfectly on the outside of the garment, and I had to unpick this bugger three times until I figured out what to do! When you sew up the second side of the zipper, put a pencil mark on the tape where the seam line should be (as shown), and then baste the zipper on ONLY WHERE the seam line hits, about 1 1/2 inch. Then zip the zipper and see if your seam lines match! it is so simple! This way you don't have to unpick the entire zipper over and over until you get it right, and it is so much easier to sew the entire zipper in after that because it is already basted where it needs to hit. HALLELUJAH! Just pinning the zipper in place doesn't do much, because the zipper will shift when sewing. Basting is where I want to be! It will save your hair from turning prematurely gray!
See here: a perfect IZ with seam lines matching.