When I'm having trouble writing, I do something else. Something mindless. I give my subconscious a chance to work on the issue. I may go for a run, take my dog out for a walk, channel surf on TV, peruse news sites on the Web. Sometimes I clean my house. While I'm doing any of those things, I think about what I want to write – in very general terms. What usually happens is one sentence or a snippet of dialogue will pop into my head and that will knock the block out of the way. I think the worst thing to do when the words aren't flowing is to panic.
I'm not really a disciplined writer — I don't have a set amount of time per day or a set number of pages. I may write nothing one day, and 50 pages the next. But I'm always ready.