For books that I've purchased, they go on my kindle, so I always have plenty of books on there. For books I haven't purchased,I use goodreads to get recommendations and I have a "to-read" shelf there. I also send samples to my phone's kindle app, so that I have a list of books I'm interested in. I can then read some of the samples & weed out those I'm not interested in. I also have a "saved books" list on my library's overdrive website.
I also agree with those that just wing it. I try to keep track of books while browsing so I don't forget them, but I never know what'll strike my fancy next.