You certainly have to have a good rhythm going and usually a little off beat helps too. Don't force it though, just lay back and groove - be funky.
Pauses/rests are very important, as are changing up your note values (half notes, 16th's, etc).
Lots of chromaticism, and/or minor scales.
Play along with the kick drum, and give some space for the snare.
I like to try to always play on the "1", whether its the beginning of a phrase, the tail end of it, or somewhere in between.
Listen to Dr. Dre's beats - most of those were recorded by real bassists in his studio, but he sampled their playing. But he, IMO, has some of the best basslines in hip hop. Other notable artists with good hip hop basslines are Outkast, Jay-Z, Tupac (with or without Dre), and A Tribe Called Quest.
Listen to tracks produced by Raphael Saadiq and J Dilla.
Also listen to D'Angelo's Voodoo album, and other NuSoul basslines by Pino Palladino.