Sharon Schamber has some great videos on YouTube that demonstrate blocking. She shows extreme examples, but the same techniques apply for regular blocking. Here's a link to one of her videos. If you do a search on her name, you will find lots of others:
http://www.youtube.com/watch?v=v-3RIWhBvcA
Edit: I see you mentioned squaring blocks rather than blocking them. If you are making blocks a specific size and they are a little wonky, blocking is in order.
There are techniqes for making a block bigger and then cutting it down to size; that would be "squaring". There are probably YouTube videos on how to square a block too. Some blocks cannot be cut down to size because you destroy points, etc., and that's when you "block" instead of squaring. Sorry if I have answered a question you weren't asking!