A code kata is a simple programming exercise, typically no more than a couple hours long for the average programmer, that can be done in multiple ways using multiple languages. I've been using the code katas from [http://codingdojo.org/cgi-bin/wiki.pl?KataCatalogue] to relearn C.
So far, I've completed KataFizzBuzz (and the extension), KataMinesweeper, and KataPotter. I've had to use a reference book or two for many of the simpler portions of the language, but that's the be expected when you haven't programmed at all in a couple of years and haven't programmed in C for several years.
Overall, I've happy with my progress. I've relearned a good bit and will tackle more difficult problems after several more katas.