Expect to spend quite a long time fixing the sync. Keep things short. This makes it easier to nudge elements when necessary and eliminates long-term sync errors. Split dialog into files of no more than a few minutes. If you're working with music, think in terms of overlapping cues, rather than a full score.
Check sync by putting one reference on the left speaker and the other on the right.
When they're not in sync, the error will catch your attention immediately. Bring your mix to an audio post house as the final step. We've got ways of stretching and compressing things to make them fit. If that's not practical, transfer the mix to mag stock and make final adjustments on a film flatbed editor. For heaven's sake, don't forget about the 0.1-percent slowdown when you transfer film to NTSC video.