Now this is Marketing! by Russ and Lori, on Flickr
When you decide to start a blog and have the intention of it becoming a high traffic go-to place for people, it is generally good to focus down on a niche, or target market if you will. When you do this, the quality of people you will attract to the site is much higher, and since it is deliberate, you do not need to spend as much time doing it.
I think the best example of this from the work I have done is a site that I created, using the Ning platform, for Mother's diagnosed with Multiple Sclerosis. It was a niche that I found accidentally, a mom I was chatting with mentioned not having a place to go to talk to other moms with MS and when I did a pretty thorough search online I agreed! There seemed to be nothing there. I saw parents with MS, I saw Parenting forums within MS sites, and that was about it! Later I saw it as a small sub group of mothering sites, but there wasn't a high number of members, probably because the moms at the particular sites were already pretty filtered down to that sites niche.
So back to the story, the site launched and it has been booming ever since, it's been mentioned 2 times in national publications, once in a magazine and once in a newsletter, and also in 2 local publications, a women's magazine and a local chapter mailing.
Easiest way to find your niche is to look around you, talk to the people you know. Think of all of these things and hopefully it'll come to you.
If it still doesn't come to you then just start blogging, as much as you can, and after a week or two (more if you haven't written much) look back over what you have written and see if you notice any trends in your writing. Once you do, you can focus on that.
Obviously if you have a business that does something in particular like a bookstore you probably already have a niche pretty well carved out, your local area + books, but think about what kind of books, what kind of people, maybe it will be about mystery books and tea (might want to serve/sell it too if you do that!) and cats if your store has one of those cats in the window that watches over everything. Something like that gives you both flexibility and also a focus for your blog to fall back on.
So what is your niche?