there are a lot of tricks you can try. if you have a steamer available, use that (i did). or you can use a heat gun to soften up the adhesive in small sections and peel it back. the easiest way would be to just take a razor blade or plastic scraper and flip the film back enough until you can get a good grip on it and slowly pull it down at an angle. ammonia, rubbing alcohol, Easy-Off, 409, or soaking it in Windex will take off the rest of the gunk left over. get creative. lol lay a cover down beneath the window unless you don't mind your interior getting dirty.
we have this really good cleaner we used at the shop when tinting...i don't remember the name, but it cleans up the leftover residue really well. i'll check it later if i remember.
anyway, it sure beats paying someone $150 for something that'll take you an hour max. to do yourself! lol