1. The commentary is running faster than the movie
If you live in North America (and even in some other countries like Japan) chances are your DVDs are NTSC - also known as Region 1. NTSC is a broadcast standard and basically involves the frame rate of your DVD; in other words, how fast the moving pictures are actually "moving" from one frame to the next. If your MP3 commentary seems to keep getting ahead of the movie, and you have to constantly pause and resync, then chances are you're using the PAL version. (Which is not what you want, in this scenario.)
Solution: Just make sure you're using the NTSC format of our commentary MP3. You can re-download your files as often as you want, so head back to your Library and get the correct version. More information about DVD region codes can be found at this Wikipedia article.
NOTE: If you are watching along with a Blu-Ray, you'll also want to use the NTSC version of our commentary, as Blu-rays tend to move at the same rate as the original source movie.
2. The jokes don't make sense to what's happening in some scenes
Some filmmakers just make too much movie for their initial theatrical release, and so later on they put a "Director's Cut" or "Extended Edition" to help
Solution: Check that riff's product page for notes regarding whether we've used a Theatrical or other edition of the movie, and make sure you're watching that version. We include a link on each product page to the correct version of the riffed movie on Amazon for your convenience.
3. Watching a stream (Netflix, Vudu, etc.) or an iTunes download of the movie and losing sync
Because of the multitude of streaming services out there, it can be difficult to ensure sync with all of them. For instance, Netflix Instant's streaming version of The Hunger Games moves just a hair too fast compared to the DVD; this causes our MP3 commentary to go out of sync.
Solution: Unfortunately there's no easy way around this, and you'll probably need to track down the DVD instead.
4. My movie came from a questionable movie source, and I can't get it synced up
If you "found" your movie somewhere in the dark recesses of the interwebs, there's a possibility that whoever encoded it did so at a different framerate than is standard for movies. If that's the case, you might find yourself an official copy.
Because RiffTrax commentaries are recorded to the officially-released DVD version of the movie, we can only guarantee sync with it.
Solution: We highly recommend getting the DVD and trying again! (And hey, consider making a donation to make things right.)
Submit a help ticket if none of these apply or you continue to run into sync issues, as that might also mean a new version of the DVD has been released, and we might have missed it.