Today I had to remove the window tint from a car I just bought so that it would pass Utah's safety inspection. Two tips, which are relatively obvious but helpful:

1) Use a heat gun or hair dryer. The film will peel off much easier that way. Most modern tint film will pull off in one sheet if you get a little heat on the glass.

2) This was the one that surprised me. Once you peel the film off, there will be some glue residue stuck on the window glass. I started out using 3M adhesive remover because I already had some, and it seemed like that should have been the best thing for the job. But it just made the residue gummy. I found a tip online that suggested good old Windex, so I tried it. 10 times easier. The windex keeps the residue wet, unlike the adhesive remover, and keeping the residue wet and soapy with the Windex made it much easier to scrape off with razor blade. Using the Windex it didn't take too long at all to get both windows cleaned up.