I usually use a gray or black primer before painting, but I have had success without priming as well.
When I prime, I usually use a lacquer primer. For a long time I did not prime, and things came out just fine.
Just make sure you clean the plastic completely. I use a mild soapy solution, and rince it thoroughly. Let it air dry, then the paint should stick well.
www.naplak.com/modeling ... a free site for modelers
www.scalehobby.com/forum/index.php ... a nice Modeling Forum