Soloing in general is a skill to be learned. And the starting point (even though I'm a bit of a theory geek) isn't scales, modes, chords, etc. It's ideas. So, JimmyM's suggestion is where you start. He says "listen to good soloists and steal their licks". That's pretty much how every great soloist started.
Now I'll add a few warnings. First, he says "listen". That means put down your freakin' bass, turn off the phone and the computer, and LISTEN with your heart and ears to good soloists. Sing along with what they play. LISTEN to how the notes in the solo interact with the chords, LISTEN to how the phrases are built, LISTEN to how the phrasing of the solo line interacts with the rhythm section, LISTEN to the shape of the solo- how it starts, builds, and ends.
Second- he says "... good soloists". That really means "don't listen to bass players". Great soloists are great no matter the instrument- it's the music they play that makes it great, not the technique on a specific instrument. So, listen to really really good soloists- and that means horn players and singers generally, not bassists nor guitarist (I'd make an exception for Joe Pass, Mike Stern, and possibly Larry Carlton on guitar- and Ron Carter, and maybe Abe Laboriel on bass). Listen to Miles Davis, Louis Armstrong, Ella Fitzgerald, Frank Sinatra (the master of phrasing), Wayne Shorter, Bill Evans, Oscar Peterson, etc.
Next JimmyM says "steal their licks" and that's a can of worms. Yeah it starts with licks- but the point is to cop what they're doing- but also learn WHY it works- that's where the study of theory starts coming into play. Stealing licks is a process where you'll first learn to play exactly what someone else played- that gives you both the physical approach, AND the music. Study how they used the notes against that chord- and extrapolate to other chords.
Now the next step is the hard one- Take a bit of music over which you're going to solo and record it. Then lock up your bass and listen to that bit over and over again. Just listen to it. Then after about 20 times, (with your bass still safely locked away where you won't get to it), play that bit of music back and SING what you think would be a good solo over it. Do that a couple of times, but not too many. You want it to be fresh, but not neccesarily the first thing that pops out of your head. Get to where you think you have a decent (not great, you're just starting) solo thought up and record yourself singing that solo*.
Then sit down with your bass and that recording and learn EXACTLY what you sang. Not just the notes, but the dynamics, the flow, the phrasing, the pacing, the whole expression. Now THAT'S a solo that's all you. You let the music guide your fingers, instead of the typical bass solo which is the fingers randomly twitching until the drummer hits a rim shot...
John
*Don't worry if your singing sucks. My singing was once described as "imagine Yoko Ono and Linda McCartney singing a duet". The point is that you'll know in your head what you were going for and the recording is just a reminder. Besides, the more you do it the better you get at singing too, and THAT'S a much more useful skill for a working bassist than soloing.
jte
Ive got great pocket (if I don't say so myself
) but I need a little bit more to say during improv solos. 
