Here is my technique. Using the installation marks on the belt and two black binder clips, you will get it right every time, if the crank sprocket is also aligned correctly. Even if the cams rotate from TDC, if the mark on the belt and the pulley mark remain aligned, you're good. Once I tried this, it went on correct first try. (you can't see the mark on the pulley--it's under the clip.)
You can even do the same thing with an existing belt, as long as you put marks on the belt before you take it off. If your new belt has no marks, then mark your old one and then use that one to mark the new one.