Use natural tonal enhancement when playing rumba on the bass. A good technique is to vary the location where you pluck the bass strings. Plucking the strings closer to the bridge will give you a sharper tone with a treble edge. The strings are tighter there. Playing near the pickups produces a richer, deeper tone. Pucking notes over the fretboard deepens the tone even more. By alternating where you pluck the strings, you change tonality that supports the percussive feel of the rumba.
Play "walking bass" notes in between the emphasized notes. A walking bass line is three or four notes, usually the first, third, and fifth in a key, played evenly to create a "walking" effect. By playing this type of line in conjunction with the emphasized notes of a rumba, you strengthen the dance feel of the tune.
Make use of syncopation (sharp alternation between playing and not playing) techniques to create movement and space. This technique creates the solid dance feel and rhythm of the rumba while leaving room for other instruments to play lead lines. The role of the bass in rumba is to support the band. Use the side of your hand to slap out a beat or mute notes to achieve this effect.
Practice the slap and pop technique. This is accomplished by pulling the strings away from your bass and letting them pop back against the fretboard and by thumping your thumb against the strings to create the slap. These techniques used in combination are often found in rumba and can be used effectively if not overdone.