1. Use alot of arpeggios. Although bass can't play chords, when you play chords broken it sounds really good on bass. SO skip through some arp patterns.

2. Use pedal tones. Hit an open A while playing some crazy licks on the G string.

3. Use larger interval skips in runs. Unlike guitar where you can just run up and down scale steps, the bass is prone to more skips in 6th's and octaves. So use 'em.

4. Most important is bass is a rhythmic instrument so the more you vary the rhythm the better the solo will sound for bass. Add rests, and off beat pops.

Those are my tips for a bass solo opposed to a guitar solo. Everything else that has to do with guitar soloing will fit bass. Ex. like tapping, sweeps.