It depends, if I can't seem to get any ideas about blogging or writing, I look for new ways to become inspired... for example, my blog is political and when I am short on ideas all I have to do is watch the news, listen to my favourite news-talk radio station and listen to what callers are ranting about, or pick up a book I haven't gotten around to reading like Hayek or a political biography for example. I usually begin having ideas by the next day.
If I have ideas in my head, but when I sit down at the computer and just see a blank screen, it's usually because I'm trying too hard in my mind to write the perfect piece. The best advice for this is to just start writing and don't worry about whether it sounds bad or how it turns out. You don't even have to write complete sentences, you can just jot down words or phrases that come to mind that you want to write about.
I also like listening to music to help me relax and if it gets the juices flowing, I like to write them down (brainstorm) for further development later.