Keep it simple. Bass lines in many salsa songs support the piano and guitar. The key function of the salsa bass line is to create a groove. The groove is a rhythmic pattern that creates a musical feel, and the bass in a salsa number creates this groove by playing only a few notes strategically placed. This strategic placement is often on the backbeats of the song, or the unaccented beats of the song. The bombo, or the up beat between beats two and three of a salsa number, is the key note to emphasize in a salsa number.
Practice slapping and popping techniques. Slapping and popping the strings of the bass provides a funky feel that fits well with the salsa style. Slap the strings by using the edge of your thumb (some players use the edge of their hand). You pop the strings on your bass by pulling the string away from the fretboard with your thumb and forefinger and letting it bounce back against the fretboard. A combination of the two techniques creates a nice dance groove but should not be overused.
Understand the fundamental makeup of the salsa. Its key feature is a construction known as three-two, which uses a measure of music consisting of three strong notes followed by a measure featuring two weaker notes. This provides the dance feel of the salsa and can be identified easily when you know what you are listening for.
Listen to as much salsa as you can. Play along with salsa recordings and try to imitate the styles you hear in the songs. The more you play, the easier it is to recognize the salsa rhythm.