Here are a couple of tips for lipsyncing in flash that i've picked up from friends.
Brian Ellis has a method wherein he will do a pass in which he will just lay down his plosives- that's his p's m's and b's- basically the closed mouth sounds- as well as his F's and L's. Since vowel sounds and rrrs and ooos can be a bit more floaty, he does a second pass where he lays those in. I can attest to how fast this can make lipsync go.
Lars Edwards showed me how you can put in a mouth shape as much as two frames before the actual sound, and it still works. Two frames behind looks like mush. So rule of thumb is lead the sound, don't sync behind the sound.
All the stuff i've worked on to-date has had a rule for vowel sounds: pop open and tween closed. Depending on how many mouth-shapes you've built, there's wiggle room which renders this a soft rule, but generally it does tend to look better.
See my crazytalented friend jo's advice on lipsync HERE. Seriously.