I'm also a big fan of Marty Schwartz's, and I've learned a lot of great Blues licks and soloing techniques from him. Most recently I've been working on combining major and minor pentatonic licks and runs into my improvisations, and I'm loving my progress. Thanks Marty!

I love YouTube, I really do, besides great resources like justinguitar and Mary Schwartz, there are a huge number of excellent backing tracks you can use improvise over, here are a few of my favorite favorites

Happy Blues in E (great for mixing major/minor pentatonic) http://www.youtube.com/watch?v=Dxo2bEX45KI

Acoustic Rock Style Backing Track - D Major http://www.youtube.com/watch?v=ohbtYFRe-xc

Marty teaching 'Fly me to the moon' (great jazz standard) http://www.youtube.com/watch?v=lr3fa1NDVB4