Just did my first 24-hr mtb race this past weekend, the 24 Hours of Allamuchy (see www.24hoursofallamuchy.com for more info), and I have to say it - what a blast!
(I know our pal Dizzy did her first one earlier this summer, and she also thought it was great!)

I was fortunate enough to be on a 4-man team with 3 experienced 24-hr racers (one had done one other race as a 2-man team, the other has done several as a 2-man team, and our captain has done numerous races as a solo), but there were many teams there for which this was their first one.

I would say that having some mtb racing experience would be a big help (we were all Expert-class racers), but is certainly not mandatory. I think that camping experience is equally important, as it is absolutely key to have as plush a base camp setup as possible. The most important thing to have is a good, upbeat, positive attitude.

What to bring:
1-5 of your best friends/riding buddies - If you've never done a 24 hr mtb race, it's probably not a good idea to do your first one as a solo. The more the merrier in terms of social scene too, and you can always bs w/ the other teams that are camping near you. We met a lot of really nice/cool folks this weekend.
Make sure that at least one team member, preferably several or all (or at least one support crew person) is experienced in bike repair. You WILL be repairing your bike. A lot.

LOTS of food/drinks - you need to eat and drink a LOT in between laps, even if you don't feel like it. And you want a variety of foods (sweet/salty, hard/soft, hot/cold, etc) to choose from, so you can find something that appeals to you.

ALL of your bike-related clothing and gear - you will want to put on fresh shorts, jersey, and socks for every lap, if possible. If you have spare shoes, gloves, helmets, etc - bring them too.
More than one camelbak - bring it! (my buddy accidentally melted his by leaving it too close to the fire while drying it out - ooops!)

LIGHTS - you need at least one good, powerful lighting system, 2 is better, and chargers and spare batteries for the above are essential. (my "fully charged" main light died after only about an hour, it should have lasted well over 2 hrs, leaving me to ride (and walk, a lot) for a few miles over slippery, technical terrain by the light of my Petzl Tikka)
If you have 2 good lighting systems (bar and helmet) then you can go faster, and will still have one good one left should one die out on you)

Spare parts/bike - you never know what can or will happen in a 24 hr race, so you need to "be prepared". I brought an entire extra bike (my old beater - to cannibalize for parts if needed, and to ride if necessary) as well as lots of tools/lubes/parts for my bike.
Chain lube, brake pads, spare shifter and brake cables, and extra new tubes & tires are key. We had a bike repair stand too, it made all the repairs (did I mention you would be fixing your bike a lot?) much easier to do.

Camping gear/shelter/comfort - this is car-camping, so ditch the 6 lb, 3-season tent - you want the Barnum & Bailey Big Top tent!!! We had 2 BIG tents, and a pop-up awning (our bike storage/repair area), as well as a large tarp over the picnic table. Probably had 4-5 coolers between us, a coleman 2-burner gas stove, a charcoal grill, lanterns, folding camp chairs, lots of water, etc. Make it as plush and comfortable as possible, you will be spending a lot of time there, in all kinds of weather. Our "compound" would have been the envy of the Kennedy's!

Ride on!
-Mike B.
"Flailing?" "Flail on!"