Types of clay to use---and not to use? : Head Sculpts

I was going to go shopping for clay next week, and starting my sculpt soon after. However, I have hit a small dilemma...I have no idea what kind of clay to use! What type or brand should I use, and what works best for casting in plaster (cleans out the easiest)? If possible, I would like to get something I can use more than once OR has a long shelf-life.

Anybody who has done these types of heads, I would love to know what you used and what you think the pro's and con's of it were. :3