Everything that can be said pretty much already has. I'll just add what I learned from my own experience. First, make your answer as clear and organized as possible. The prof probably does not enjoy reading 100 essay answers, so the easier it is for him/her the better. Second, don't take anything for granted. My Contracts I prof told me that a lot of people forget to cover the most basic rules (offer, consideration, acceptance) and just go straight for the main issues in the question. So don't take anything for granted. You gain or lost points for every issue you spot or miss. Also, don't get distracted by irrelevant information. You're not going to care about all of the info in the question - just focus on the actual issues and apply the relevant rules (the kitchen sink approach is not going to help).
I'm a huge advocate of practice exams. I do them for every class, so that by the final I already know how I would approach a question about pretty much any topic that was covered. One of my profs actually offered to look over practice exam answers and provide feedback. I took him up on it, and he said I was the first person who ever did. I got the highest grade in the class on that final, and I think it was because of the preparation I did. Know the rules and understand how to apply them efficiently. What else can I say?