If I'm lucky (and the convention is well organized), by late January I might begin to get panel topics for the February and March conventions and can start to prepare for panel discussions. For some topics I know those well enough and know the other panelists well enough that preparation is just a matter of having a few talking points to bring up and discuss. I've learned to have those written down beforehand so I don't suddenly draw a blank in mid-discussion and/or forget one. For other topics I might have a list (especially those panels on trends in YA or new titles/authors to know about). One of the advantages (or disadvantages) of being a librarian in my other job is that it's very easy for me to research a topic and come up with a useful list. Sometimes the list is long enough to be a handout that I can pass out to interested people at the end of the panel.
If I'll be doing any readings of my works at a convention, I usually decide on which pieces to read once I know how much time I'll have. Sometimes I'll be reading by myself, but other times I'll be part of a group reading, usually a Broad Universe Rapid Fire Reading. I'll usually practice the reading a week or so before the convention to know if it will fit the time period and to spot where I might trip over a phrase.
How does one find a science fiction convention? There used to be several lists of conventions, both online and in science fiction magazines such as Analog and Asimov's. Those lists have slowly disappeared. My friend Lin Daniel has begun one at http://www.lindaniel.com/conventioncalendar/. You can leave it at all genre, or it can be broken down to categories like anime, filk, sf, or combinations of several genre.
There are other posts at this blog (several by Gloria Oliver) as to what can be seen at a convention and why authors should consider attending. Here's Gloria's top 5 reasons. Promotion is one good reason. Networking with other authors, as well as editors and publishers, is another. I usually find at least one or two panels that I'm quite happy to be in the audience and learn something new.
Do you attend conventions? What preparations do you make?