Here at the library we use a database called the KDL What's Next Database, see http://ww2.kdl.org/libcat/whatsnext.asp
The other way we try to help patrons identify books in a series is by using the call number sticker along with a colored sticker. So, for instance if you want to find the next book in a Nora Roberts series, look on the shelf for the colored stickers that match the ones on the book that you have already read. We try to start off each author without a colored sticker and then in suceeding series we put a colored sticker on the book. In the case of Nora Roberts, she has written so many series that we have had to use two colored stickers to identify the series. We also put the book number on the call number sticker.
Maybe an example will help to understand:
If you want to find the next book in the Three Sister Triology by Nora Roberts, notice the stickers on the spine: