Analyze the problem. Is it that you can't play a solo, or that you can't create one? Two very different things. Bottom line is to solo well, you gotta have good ideas, and only then do you have to figure out how to execute those ideas.
So, first of all, ignore bass solos. Why? Because most bass players aren't very good soloists! Learn solos from master soloists. Now if you're talking about improvised solos, that means get into jazz. Learn from Miles, Satchmo, Dizzy, Oscar Peterson, Coltrane, Dexter Gordon, Joe Henderson, Roy Elderidge, Ella Fitzgerald, etc. Avoid blues guitarists (as much as I love them, they ain't good ways to learn to improvise solos because they're much more about licks and a limited sonic pallet). The only really great soloing guitarists I know of who you can really learn a lot from (note- I'm not say these are the only great soloing guitarists!!) are Duane Allman, Jeff Beck, Joe Pass, Mike Stern, Barney Kessel, Larry Carlton, and John Scofield. Also, learn melodies. Learn the head to standards, learn the melodies to Broadway tunes, learn folk songs, learn Lennon/McCartney melodies. Listen to classical music.
Let all that stuff simmer around in your head and your ears. Now, the mechanics of this on bass... Record yourself SINGING a solo over a progression. DO NOT even hold a bass in your hands when you do this. The point is to get the music you HEAR in your head recorded.
Then do the hard work of sitting down and learning to play exactly what you sang. That's the music in you, and its not based on bass licks and where your hands want to go. That's how Otiel does those things with ARU, and how Mike Gordon knows how to steer Phish. It's also how Jamerson came up with those classic lines, and how McCartney comes up with those melodies under those great vocal melodies. It's all the same process- have great ideas first, then learn to execute them.
And you get great ideas by exposing your ears to a huge range of musics.
John-