The read-a-thon is only a week away now, and I can't wait :)
If you decide to participate in the read-a-thon...
DO decide whether you want to do it full out or not ahead of time and arrange accordingly. There's nothing wrong with deciding to just to a 1, 2, 5 or 12 hour read-a-thon instead. No reason not to join in just because you might have a previous engagement that prevents you from reading the entire 24 hours. I've never once gone without sleep, and more often than not I've had to end early because I had church in the morning.
DO plan ahead. Let your family know you're not to be disturbed except in case of an emergency, figure out approximately which books you want to read and where they are, be sure to have stocked up on snacks, and know what you're going to do for dinner. I love my crockpot for this, as I can cook dinner before the read-a-thon even starts! Having a family member you can coerce to cook dinner for you or go get take-outs works too.
DON'T plan ahead. Yes, I'm contradicting myself, but while it is a great idea to have a general idea of which books you want to read, so you don't have to go hunting for them, don't have a set stack of books you HAVE to read during the read-a-thon. Anything can be a chore if you have to do it - even something as pleasant as reading - and in order to keep up your mojo for the entire read-a-thon, it's important to allow yourself to read where the mood takes you.
DO figure out what motivates you to keep going. For some people this may be the satisfaction of finally reading that 1000 page chunkster that's been standing on your shelf for ages. For me, it's finishing a book and being able to move it from my "To-Read" pile to the "Read" pile.
DO make a trip to the library ahead of time or in other ways stock up on "light" reads. You want a book that you can power through without too much effort. The books don't necessarily have to be short (see above - you may decide to go for one 1000 page book rather than five 200 page books), but you should stick with books that make for easy reading. Some chunksters I've happily read in a day include the later Harry Potter books, Phantoms by Dean Koontz, Paganini-kontrakten by Lars Kepler, The Girl Who Played With Fire by Stieg Larsson and Eclipse by Stephenie Meyer - all 500+p books.
DO take a break from sitting down every once in awhile and move around a bit. You're going to get SO stiff otherwise. Notice I didn't necessarily say take a break from reading - you know best yourself whether or not you're able to read and walk at the same time ;)
DON'T go nuts on snacks. I know it's tempting, but too much sugar is bad for your concentration.
DO remember that audiobooks and graphic novels also count. Variety is your friend.
DON'T feel bad if you get sidetracked being a cheerleader for other read-a-thon'ers or by doing the hourly challenges. Just like NaNoWriMo, the read-a-thon is all about taking a solitary activity and making it social :)
DO have fun. I know I will :)