I have a short piece on how to write a philosophy essay (pdf), and a short piece on writing clearly (pdf). I hope that they're of some use to students. If you have comments on these pieces, please email me.
Jim Pryor has some excellent advice on writing philosophy essays here. If that isn't enough, there's more advice by Peter Horban here. I personally prefer the Pryor piece, but reading both can't hurt.
Larry Trask has an excellent guide to punctuation here.