Well as a professional writer I can tell you we have it easy...sorta, the ideas aren't always there but the lucky ones can pull one on of their hat sorta speak, but I basically punch a clock for writing, from 10 am - 4 pm I sit at my desk and I write, whether its good or bad, or has absolutly nothing to do with my current project I write, usually by days end I have something worth while, although occassionally I end up with a waste of 6 hours. Its not a hard thing to do, just let your imagination run wild and learn to type as fast as you think.
I've had an entire week go from a Fantasy Novel full of magic and mystery into a murder full of suspense and terror, I just go thorugh everything I write and pull out the parts that don't fit and use them for filler for a later project that it will fit into.
I do suggest though that if you work on multiple projects at once have them be as different as possible and as different as you feel comfortable with.
As for recharging your battery, TV and movies are good, although I find physical activity such Hiking, biking, swimming, to be the best...personally I go to the gym for a couple of hours each day...or even just a drive out in the country with the music blarring so loud that you can't think about your projects. The trick is when your not at your desk, don't write. It gives you better sense of being "at work" if you don't, I write in one room, and when i'm not writing I'm not in that room, not always an option for some people I'm just lucky to have the space.