I'm no expert on improvisation but I have read a few articles on some of the Jazz greats. They ALL say that the best way to develop your improvisation skills is to listen to as many recordings of different players and styles of jazz as possible and try to copy. After a while you'll learn what works for you and you'll develop your own style of playing. You could also try using some of the jazz play along CD's which list all the changes and turn your practice sessions into a jam session!!
If you get the chance, listen to some of Clarke Terry's recordings. He's my favourite jazz trumpeter and quite honestly is a lesson on how to play the trumpet .....full stop! I was also completely starstruck when I heard Wynton Marsalis live with the Lincoln Jazz Centre Big Band. They did a tribute to Louis Armstrong and Wynton was truely amazing (though it's widely known that he could mimic Armstrong even when we was 12 years old).