The 6-bit balanced code Phillip suggests is a good one. RF Monolithics has an appnote with a 6-bit balanced code in it:

http://www.rfm.com/corp/appdata/AN43.pdf

Another balanced code to consider is 4B/5B NRZI, which is a bit more "dense", in the sense that 8-bits expand into 10 bits rather than 12. Paste the following Google URL w/ search parms into your browser to read more about it if you are interested.

http://www.google.com/search?hl...22+%2Bnrzi