My TBR for my Kindle is
a list of the mostly freebies I've purchased for it, in the order I bought them. I just pick the next one in the list.
For my Sony, using my PRS-505 that is loaded up with as many unread epub titles as I can fit, I go into the title list and pick one of the number buttons, and then I select the title at the top or the bottom of the page. It is a sort of random selection and if I didn't do this, I think I wouldn't read any of the "classic literature" or other freebies that I have collected. This is only if I don't have a particular title in mind that I want to read instead. Then I download the book to my T1.
When I thought the 505 was dying, I was at a loss about how to pick out my books in the future.
I also use my 505 at work for lunchtime reading and for that I am working my way through a particular, light-toned series.