I like 37signals think like a chef metaphor. Look at the food network. New chefs, new cooking shows all the time.
If you were creating your own "cooking show", and someone was going to film you doing the things you do, what do you think your show is going to be about?
What is it that you do every day for work, for fun, for your business, that you think you can teach others. There's likely something that makes you unique, that other people can benefit from.
Sometimes you can look to your customers or site visitors for inspiration. What sorts of things do they like? What topics do they find interesting? Once you start blogging, you'll find that certain topics resonate with your customer base.
In the very early days of our startup, we didn't have a clear picture of what our final product would look like, but we knew what problem we were trying to solve. So we wrote (and continue to write) posts about the issues our potential customers face.
When you're a startup, nobody knows who you are or what your solution does. But most people have some idea of the problem they're trying to solve.