Hi there. I guessing that when you say improvising you mean soloing. There's some great exercises for this but it depends where you are as a player as to which exercises you tackle. First of all, get a metronome... they never go out of time and are a great reference when we practice/improvise along with them.
Many bass players find that when they start learning to improvise they have a problem with time and knowing where the one is. Remember, when every else is soloing there's a lovely bass line underneath them spelling out all the chords and making them sound great... hopefully! The gag is, when you come to solo the lovely bass line disappears and all you might get is some ambient type chords from the keyboard/piano or guitar! So bearing all this in mind bass players, especially in a jazz context where they are navigating chord changes etc have to have a really strong internal pulse.
A great way to practice this is to take a tune you already know, lets say autumn leaves, then get the metronome and set it to 50bpm. Your now going to improvise along with the metronome through the chord changes making sure the metronome is on beats 2 and 4. This is tricky to start with but really worth the practice time as it builds your internal pulse by putting you in a solo situation where you can't rely on anyone but yourself.
After you can do this raise the tempo up.
Practicing with programs such as band in the box is invaluable but sometimes gives bass players a misrepresentation of what soloing with an actual band is like. If you can solo with only you and the metronome you'll be well on your way!
Also, if the band are going crazy underneath your solo... a bit too crazy! Assuming these are guys you regularly play with don't be afraid to tell them to spell out the chords a little more etc. I was in a situation recently where i had to take a solo and the tune had multiple time signatures, most of which were changing every bar. In the rehearsals i was very quick to say to the drummer and piano player to spell out the time and chords in a more obvious way, as not only was i having to deal with bars of 13/8 and 5/4 there was also some really challenging harmony to navigate through.
Good luck!
Scott.
http://www.scottsbasslessons.com