Months -- even years -- later I've returned to them. I've had a burst of enthusiasm for a while, adding lots of new content, then stopped again.
I've often thought to myself that this was a waste of time, and that I should just backup all the content and take them offline. But then I think, no, not yet. I'll leave them up there a while longer ...
Don't delete old blogs
Ultimately I think this is the best approach. You should always keep your old blogs online. That way, you can always come back to them and start anew. If you give it time you'll have different ideas when you return. You can regain your enthusiasm about them and become more creative again.
By looking through all those old posts you'll realize how much you've learned since writing them. This will spark ideas for new blog posts. Or you can rewrite many of the old ones, including all the new and improved information.
Blogs are "public notepads"
A friend once said to me that a blog is like a public notepad. I've always remembered that observation. I think it's very true.
It's worth looking at a blog as a process rather than a destination. A book is like a destination. It's finished, complete. But a blog is much more fluid. It is a work in progress as opposed to a finished product.
That said, you can always turn your blog into a book -- then sell that very book off the blog that spawned it!
Free blogs can be advantageous
Because blogs are inherently fluid and changeable using free blog platforms like Blogger is the best approach. You can then rack up a whole lot of content on several different blogs. You can let some of them become dormant, then come back to them when you're good and ready. And you won't have to spend any money doing this.
If you use self-hosted Wordpress blogs in this way, though, you won't be able to do this. If you have three or four of them it will just become too expensive.