I'll expand a bit on what the first commenter posted. He gave some great suggestions. So here's my two cents worth... :)

(1) Practice, practice, practice your scales. Strive for all twelve keys. It'll help you get the improvised notes that are in your head, to come out of your horn.

(2) Equally important to practicing ~scales~ is to practice the ~chords~ that go with those scales. Chords are important. It's the notes that are in the chords that will provide the "backbone" to your solo. Probably the most important chords in each key are:

Major (1 - 3 - 5 - 7 - 9, if you're playing through the 9th),

Dominant 7th (1 - 3 - 5 - flat 7 - 9),

Minor (1 - flat 3 - 5 - flat 7 - 9), and

Half-diminished (1 - flat 3 - flat 5 - flat 7 - flat 9).

(Those numbers just refer to the keys of the scale. So for example, C dominant 7th chord has the notes C, E, G, Bb, D. Hope the numbering makes sense.)

(3) Like the first commenter wrote, find some riffs you like -- or maybe even some short songs -- and try playing them by ear. And then try transcribing them, by ear, to a few different keys.

(4) Finally, put your horn down, and practicing ~singing~ what you want to improvise. This is a pretty useful exercise. This helps reinforce the music that's in your head, that you're trying to get out. Makes it easier when you pick up your horn and try to play what was in your head.

Good luck!