Here are a few that I haven't seen above yet:
Go for long walks (with a bit of math in mind). Don't worry about thinking about the math, just put it in your head and then walk. This is distinct from walking to clear your mind, as putting the math in there tends to get the juices flowing whether you like it or not!
Force yourself to write down questions. Lots and lots of questions...about whatever mathematics comes to mind.
An advisor of mine said that there are uncountably many open math questions out there, some of which you have the tools to solve, in contrast to the finite list of `big fish' questions we usually focus on. Every hundred or so questions, you may be able to answer something or will at least feel a bit energized. If you asked the question yourself, chances are it is connected to something you've worked hard on...perhaps indirectly.