Smoke some weed.
Yeah, it's a stupid cliche. No, it's not the 1960's anymore, and I'm not Phil Lesh or Ken Kesey. No, you certainly don't have to do drugs to find ways out of your creative holes...
but they sure can help!
Seriously though, limits are a great way to do it. Another way I've found is by switching around on basses--if you usually play a Fender Jazz, try out a Gibson with a Mudbucker. You'd be surprised how often different basses lead to new and interesting things.
(and don't forget to smoke some reefer)