This I cut & pasted for the site Jazz Advice
This particular post
http://jazzadvice.com/8-improvisation-improvement-projects-that-will-change-your-playing/
This is particularly relevant, to me!
6) Ballads
Faster, higher, louder! This is the mentality that many young improvisers adopt and the typical scene at many jam sessions. We start to practice so we can play on fast tempos and impress people with flashy licks and high notes. As a result a lot of players fall short when it comes to playing a lyrical melody or improvising over slower tempos.
To play over a ballad you need style, expression, control, and a great sound – all of the subtle aspects of musicality that get lost in fast tempos and loud dynamics. Simply put, there is nowhere to hide in a ballad. In a stark way, ballad playing exposes your true level of musicianship.
How much control do you have over your instrument? How well can you create phrases over a chord progression? How much of this music have you listened to? How well do you know this tune?
All of the answers to these questions are revealed immediately the second you start improvising over a ballad. To improve the expressive and lyrical side of your playing start by learning one ballad. Check out Clifford Brown with Strings or Bird with Strings.
Etc Worth looking at IMHO.