The principal rule to follow is this: Avoid Monster Manual 1. (Like a lot of 4E books, avoid anything where Mike Mearls is the lead designer.)

DMG 1 actually provides very good monster building advice (it wasn't followed in Monster Manual 1, unfortunately) but the maths, of course, has since been updated. As has been stated, Monster Manual 3, Dark Sun Creature Catalogue, and the two Monster Vaults are have very good examples of how to build monsters plus the maths is correct.