This is the method I use Choose a New Theme

When you have a couple of pages and posts, you can do a preview, and it will show you the theme with your content on it, without activating it, works a bit faster, but keep searching, when you have not much content yet, because when you want to change later, you will have a lot adjustments to do.

When you see here a great website, put the url in the theme detector and it will tell you which theme it is
http://www.wpthemedetector.com/

I can totally relate to this. My method for choosing my theme went a bit like this:
- upload theme, hate it
- upload another theme, hate it more
- go back to first theme, still don't like it
- find a different theme, eh-oh that will do
- change it again
- look at premium themes, become more confused.
- go back to first theme and get on with creating content (which is a whole other story in procrastination)
Good luck, just remember you can always change it going forwards, best thing to do is create content! :)

Hi
It is a very subjective process and in the end, I do feel it is a matter of trying different ones to see how you like it.
It can make it easier to use the "Feature Filters", so you only see those you may prefer depending on the type of niche you write about.
hope that helps
paul