Practice improvising with your instrument every day. This may sound like an obvious answer, but practice is the most important step to getting better at jazz improvisation. If you are currently a member of a jazz combo, let your band mates know you want to work on your improvisation abilities. Ask them to add more time for improvisation practice to your rehearsal schedule. If you're having trouble following a particular instrument during improvisation, ask to jam as often as you can with a band mate or another musician you know who plays it .
Listen to your favorite players improvise. It is impossible to overestimate the importance of influence in jazz. If you want to get better at improvising, you will need to hear how the best in the business have done it. Avoid listening to jazz albums containing multiple overdubs and other examples of studio wizardry such as Charles Mingus' "The Black Saint and the Sinner Lady." Instead track down albums like "Sunday at the Village Vanguard" by the Bill Evans Trio that feature live performances of talented improvisers working at the height of their powers.
Find new melodies over which to improvise. If you are having trouble improvising with the tunes currently making up your repertoire, adding to the list can help loosen you up. Sometimes throwing yourself off guard is the only way to guarantee your mind stays fully open to improvisation. If you mainly play standards, track down an arrangement of a popular song or classical work. Arrange one of your own favorite pop songs if you have arranging experience.