Heh, haven't done them 20+ hour stretches since high school. Did a few 16+ hours in college 2 years ago, with other high school students. I find my ability to describe and come up with new stuff drops off as I get more tired, not good.
For advice, the break idea is the best thing i've seen. Taking a break every 4-5 hours lets everyone rest. We'd usually go out for chicken or something. The players relax while the DM gets time off to think up more stuff. Switching DM's is not a bad idea either. One time we started at 6pm, next morning we took a break at 9am to run over to the game shop and buy another game that a friend ran. We have even taken short 4 hours naps, then get up and game. You can go for days gaming that way.
Some games, such as RIFTs, Kobolds Ate My Baby, Paranoia, actually get BETTER as your DM becomes more exhausted
Nowadays everyone has work so we play 4-5 hours sunday evening. I think we played 6 or so one day when we all had monday off. We're all more rested now, and I guess the games are better and more fulfilling since we are rested.
Still, making 3 or more levels in a single (long) night was pretty damned awesome. Meeeemorieeeessss......