This is something a bunch of watch guys have been doing for years but it has come to my attention that some people still don't know about this technique. So I will try to explain it the best I can.
The issue is most rubber/silicon/pvc are flat as a board and don't fit the wrist that great straight out of the package. So what you do is you get a coffee cup/soup can/glass jar that doesn't taper and you put your strap in it sideways where it wraps around the it. Then get some boiling hot water and pour it into the cup slowly so it doesn't knock the strap free. Then let it sit for 5 minutes. What I do then is to cool it off with some cold water.
Now your all done, the strap will contour to your wrist better and fit better.