Best way to improvise? Learn scale shapes, identify the key to the song and jam along!
Try to vary the music, from blues to rock and roll and hell, even some metal!

I like to jam along to John Mayer songs (feeling), Oasis songs (pick up the pace), and then G&R or AC/DC for some classic rock action!
I don't do metal cause I can't shred yet.. So I'm going for feeeeeeel!