Back to the dialogue shot! There are basically two ways of setting mouth shapes...one is very simple and actually doesn't demand planning another that is a little bit more complicated but more precise.
Do everything in Maya
This is the way that doesn't really demand much planning, you actually do it as you go. I don't know (yet) how it works with other software, but in Maya, you can import the sound file and there's a pretty cool feature there that allows you to hear the foneme that's happening on each frame. So you scrub through the timeline with the left mouse button and hear the line "sound by sound". This is great and you can easily go on listening to the sound and setting keyframes for the mouth as you go.
The problem with this method is...sometimes when you click at the timeline in Maya, the sound you get isn't the sound that will be on that frame when you playblast it: you have to playblast it all the time to make sure it's working. So it's a handy and simple method, but not too precise.
Plan it!
You can plan where each mouth shape will happen by using a sound software to see for how many frames a certain sound actually is played. There's a free software that can do that (yay!): http://audacity.sourceforge.net
Open your sound file in Audacity (or the sound program of your preference) go to View->Set Selection Format->film frames 24 fps, so you can see the time in frames. Now you see the spectrum of the sound and can scrub through it, pretty much like you'd do it in Maya. Make a table and write the sound on one side and the frame where it starts/ends and there's your plan! It's a very simple x-sheet actually.
Here's an image to explain the basics:
1) this tool allows you to select the region of the sound wave you want to focus on
2) zoom tool so you can see that section of the sound clearly
3) ok, this is the play button, of course. But if you have selected a part of the sound, it will only play that. This way you can isolate the sounds.
4) here you can check the frames where this sound is played
