I usually have to think twice about this one. "it's" may look like its friends: Mark's, Susan's, etc. However, "its" is the possessive one. "It's" is the contraction one--short for it is.
My sister recommended "Eats, Shoots, and Leaves" as entertaining reading. I'm 30 or 40 pages into it and I would get more entertainment from a book on number theory or calculus. I am noticing my punctuation more--so maybe it is good for something.