Tired. Got other things to do. Don't want to lug out that computer. Not feeling inspired. You name it, you've got the excuses too.
I usually fail to get around these excuses, but if I'm doing well, it's because I've done some ground work to get a plot going and I feel excited about the story. After all, if I'm not excited, why should a reader be? And there's another excuse: not excited. But don't you find that it's when you actually sit down to write something, anything, that ideas start to flow? I know I do.
Even if it's just getting out a blog entry, sticking with writing in spite of the excuses is something I need to commit to. No one else is going to push me. No one else is going to believe in me until I do first.