I have been sitting here designing my t-shirts (the one with angry zippers) and discussing the relationships with zippers on Facebook and suddenly I found a way how to solve this problem once and for all. Make them always straight and never wavy.
Problem with zippers is that they are made of fabric and when you try to glue them to leather they can move around from one side to another, then when you stitch them you have to control the process to not tighten the seams too much, etc. I had to find solution for that, and I did!
Here is a bulletproof way of stitching a perfect zipper.
First, find a piece of thick paper. Not just a common printer paper, something slightly heavier, probably 100-150 g/m2. I bought a few lightbulbs recently so I used a piece of the lightbulb packaging for the experiment. The thickness of that paper was probably 250-300 g/m2 which is a bit overkill but that's all I had at the moment.
Cut it to the size of your zipper. If your zipper is 10" long and 1.3" wide, that's the size.
Take double sided tape and stick 3 pieces along that piece of paper so 2 are on the sides and 1 in the middle:
Take your zipper and stick it on this piece of paper (I won't call it cardboard because "cardboard" is usually something too thick and we don't need thick stuff, you'll see why). You can calmly align it to be as straight as possible, and it will stay this way.
I did not do the whole zipper because it was an experiment, I will only sew this part of it.
Then take a piece of double sided tape again and stick it on both sides of the zipper. And stick your leather over it.
Now we can start stitching - if your paper backing is not too thick, you won't have any problems punching it with your needles while sewing.
You should not worry about over-tightening your stitching because you can't go wrong here, just stitch as you usually do.
When you finish, just tear away the paper with the double sided tape.
...and enjoy a perfectly straight zipper!
(well, mine might not be THAT perfect because it was a scrap and it was all made really quickly because I was too excited and it was 4 AM).
I will use this method from now on!
I'll be happy if you found it useful.
...and check out my courses on pattern design and photography :)