Listen to some Bob Marley, Toots and the Maytals and any good old Dancehall stuff to get the idea. I've been listening to reggae since I was a little one and the things that I notice the most are:
1. Bass plays along with the reggae drum beat (snare on 3, hi hat or some other accent on 2 and 4 I believe), so just making a beat like that will probably make your bass line sound more reggae-ish.
2. Think lead bass. It is going to have more n\motion in the notes and variations in the rhythms than most other instruments, with the guitars and keys generally given their traditional rhythmic duties. The bass line to "Concrete Jungle" by Bob Marley is a perfect example of this, it completely makes the song and jumps around all over the place. Almost a melodic instrument sometimes.
3. A whole lot of reggae bass is played behind the beat in typical bad-ass bass fashion.
4. The chord is usually outlined pretty well. All three notes of each chord triad are on the table in reggae, it's not like getting away with just playing the root in rock music.
Bass is usually the most important instrument in any band in terms of feel, but in reggae it is really exaggerated. Drums and
the other rhythm instruments usually just stick with a pattern while the bass line is what ebbs and flows and gives that magic.