I am in the same boat that you are in right now, I will be applying this fall for next year and am looking to start email profs (but I am in a different field, geosciences), but I will tell you about the advice that I have received. I think that it should be able to apply generally to people in most fields. Basically, look at what they are working at, get a good background feel for them. In the email, summarize what you like about their work, and why you are interested in them, let them know that you are looking that their program, and with them specifically. But also ask something along the lines of if they are taking on students to work with them. When you write up an email them, its fine, because it gives them time to respond to you in their own time.
Let me know if that helps at all...