For series, Amazon often has the next book in the series for preorder, so I either preorder it or add it to my wishlist, which I check periodically.
For books I already own or plan to read in the "near" future, I keep a list on my iPad. I also started a list with books I read, with the dollar amount and, if applicable, the library name behind them, so I get very happy whenever I look at it
I just sent off an application for a fourth library. The $77 I spent combined on Brooklyn and Fairfax have already saved me tons of money, less than 2 months into my membership.