I think about this Tyler Cowen question a lot:
What is it you do to train that is comparable to a pianist practicing scales?
I'm a product manager by trade and, until recently, never really been satisfied with the answers I've come up with. However, I've found one I really like.
I try to come up with two tweet length product ideas per day. I don't judge the ideas. They can be big or small. They just have to be plausible product pitches. I've been doing this for a couple of weeks now and it has been a small enough exercise that I can commit to doing it daily but big enough that I can feel myself stretching. Most importantly, I've noticed more, better ideas coming to me at other parts of the day.
Somewhat related: I really enjoyed this Range Widely post about "pouring out your lesser ideas to get to the great ones."