I have an obscene amount of notes and folders full of half-concepts, quotes, jokes, scenarios, what have you that I've been amassing over the years. Usually, if I'm having trouble thinking up fresh material on the spot, I'll browse through those and see if I can't find anything I can use. That's what they're good for and I highly recommend everyone who writes have one (or two or ten).
A lot of people say they go to other people/music/art/exercise/fashion for inspiration and this is absolutely true for many (myself included). We are constantly surrounded by sensory input which we can translate into something creative; that's just writing 101. It is impossible to be just a solitary mental island and write anything that is of any interest to others.
Er, starting to ramble...
I find that if I just sit down and start writing, without any care or regard for spelling, content, quality, etc. eventually SOMETHING will come out of it that I can use. If the words are just not coming and my mind is a blank, I take that as a sign that I need to work on something else for a while until I get my mojo back.
If I'm having difficulty with a personal project, I'll write a roleplay post or two. ;)