More importantly, is to figure out why it isn't square to begin with....was it the cutting, sewing, pressing? Cause it's one or more of those things.
A great reference book, even though I think she's a tad anal retentive (and I mean that in a nice way), is "The Art of Machine Piecing," by Sally Collins. Even if you don't subscribe to all her methods (and I don't), it at least really explains why you're not creating perfectly square blocks
But if you've got a pieced block with many parts, and don't want to risk chopping off any points or important parts of the design, I'd suggest Sharon Schamber's helpful videos on blocking -
shrinking
http://www.youtube.com/watch?v=i6aplw_tVZc
stretching
http://www.youtube.com/watch?v=sQkF02vpVuw