I had a great improv teacher when I was younger. He gave me this list of musical ideas & devices we like to hear when listening to people improvise. This paper talks about soloing in the 'jazz style', but I think it is applicable everywhere where improvisation is concerned. I also think one needs only a minimal understanding of music theory for this list to be helpful. Every time I feel uninspired and can't think of anything good to play; after I read this, something imaginative always comes to mind.