This is an EXCELLENT video about improvisation.

I particularly enjoyed the opening material about Charlie Parker. I didn't know it, but it wasn't surprising either. Here is a link to Thomas Owens' dissertation on Parker's improvisation. 

Timings for the video:

0:00 How Charlie Parker Improvised
0:35 What is a "motive"?
1:36 Parker's Language
1:56 Elan DARE intro
2:16 Myth 1 - Improv Is creating something brand new on the spot.
4:23 Myth 2 - You can't practice improvisation
4:55 Planning an improvisation
5:54 Different types improv
6:15 Myth 3 - Improvisations are unplanned
7:26 Myth 4 - Improv comes naturally to people
8:30 What can you do?
8:43 Tip 1
8:55 Tip 2
9:28 Tip 3
9:49 Tip 4
10:15 Tip 5 BYE 

There are a number of useful links on the YouTube page.