ok...The bigger problem is always rhythm than melody. You didnt even say if its slap, tap, finger or pick solo. There are some differences you know.
So at first take a scale or sth and try improvisate rhytmicaly. Solo have to flow, as A Minor said, so try to keep some simple structure in there.
If you are talking about fingerstyle soloing, then take your favourite GUITAR solos (Django Reinhardt, Bireli Lagrene, Stochelo Rosenberg - their licks sound best on bass) and try to find out what do you like about them and try to play that parts. Lets start from here.