Just write whatever's on your mind, whether it's a good thing that happened to you, a blog venting about something a bit depressing or something about what you're up to that day. Write! LOL. I usually write my blogs spontaneously so they're usually all over the place but a lot of the time, I do write my shorter 100 word blog for giving out a quick announcements like, if i'm not going to have internet access for the next few days. My more serious writing related and fan girling blogs, are posted at random and often more thought out.
I run in to the same dilemma when it comes to blogs. A lot of times I'll sit down and think about one or two things that were really exciting to me, or really important - or if it's something sad, something that really got to me and I need to vent about. Then I write a blog and post it and I don't think about it again.