Write out a story arc. Spend some time working out what you want the overall direction of your story to be. This doesn't mean you have to put down every little detail before you start posting entries, but it really helps to have an idea of where you're going with your story, rather than making it up on the fly. As a fictional blogger, you don't have the luxury that other bloggers have of being able to sit down and report the events of the day as they happened--you have to make that part up, and it helps a lot if you know in advance what you want to happen in the long run.