Okay, I feel compelled to chime in here since I'm a teaching assitant for legal writing. (1) Get intimate with your citation manual. If you use the Bluebook, learn to love (or at least learn to use) the Bluepages. It can make a huge difference. (2) Use good grammar and don't misspell words. Don't rely solely on spellcheck. Use the Redbook or whatever your professor uses for grammar-checking. When you are close to finishing a paper, read it backwards. Errors that your brain would normally overlook will jump out at you that way. (3) No legalise. Check your library for a book called Plain English for Lawyers. Learn it, live it, love it. (4) Edit, edit, edit. And then edit again. (5) Be professional. This means, among other things, turn your paper in on time. I could keep going forever, but I'll stop for now.
Aside from the legalese, I think a lot of this is common sense, or if not common sense, then stuff that we should have learned already. I think it's sad that even though we've all finished undergrad, most legal research/writing classes still have reason to mention this stuff over and over again.