I won't claim my method is any good, but what I do is keep track of every book I come across in Goodreads if I have the slightest inclination to read it. As you might imagine, this ends up being a lot of books (more than 5000 at the moment I think).

When it comes time to choose a new book to read, I'll select a category (e.g., history, fiction, etc.), sometimes just by picking one, other times by randomly choosing. Then I randomly select 20 books from that category and decide which of the 20 books I am most interested in reading. I also roll a die and 1/3 of the time I will use the same method to pick a book that I've already read to reread. I feel like it gives me a nice diversity in the books I read, but gives me enough choice where I am really interested in the books that I'm reading.

I will follow threads, too. If I liked a book by an author I may continue reading a few more books by the same author. Or if I liked the subject I may read a few more related books on the same subject.