What causes audio/video to be out of synch?

[otwist26]

I always seem to have varying degrees of audio/video synch problems when encoding DivX video (and other codecs as well). I'm getting reasonably good at fixing the problems in the resulting video, but it's always extremely time consuming.

Can anyone explain (or point me to some documentation explaining) what exactly causes this problem? and possibly how best to avoid it?

many thanks!

[Qyd]

there are many ways and tools for conversion
perhaps if you would briefly describe your routines somebody might tell you where it goes out of sync and suggest alternatives