All of the advice above is great, my problem with improve/jamming had always been brain lock,lack of creativity or something like that.I have always played what I like to call !in the box!.That being just like its heard on the radio.Playing the Holiday Inn/Ramada clubs ment you had to play top 40 dance music in a familiar way.Club owners wanted the patrons dancing and sweating which ment more drink sales' more patrons, more money for the club,If the band Could not produce you were not booked again.
I think I,m drifting off a bit but my point is If you learned this way its harder to get back to jamming. What had the greatest effect on my playing was the purchase of a good drum machine and a book on scales and finger exercise,s,Playing along with the dead also helped.You will find that when playing scales with the machine you will almost automaticly start to improv.And as Ollie says keep in meloidc.
Hope this helps
Chuck