Run It!
Start your event during a workday. Many people only browse the internet actively at work and you want to catch them before they leave the office on Friday afternoon. Not only that, but there's always a higher chance another marathon will be happening on the weekend than a weekday that might overshadow yours or vice versa.

During the event, play the games, interact with the chat, solicit donations (but don't be pushy), encourage fanart, occassionally tell people what it is that you're doing, and just make everyone watching seem like a part of the show. No one wants to just watch some zombie playing video games for 2 hours, let alone 24, but everyone wants to be a part of the party and have their name said over the internet. Make sure you have someone monitoring the chat at least some of the time so they can field questions, run polls, and make people laugh. The bottom line is, you're putting on a show and that show's sole purpose for existing is to attract viewers and to entertain those viewers. Be entertaining!

SLEEP, SLEEP, SLEEP! Be sure that someone sleeps at least some of the time. It might seem like a ridiculous suggestion considering this guide is about running a non-stop, multi-day, gaming marathon, but it's impossible to do without sleep. Your event will fail if everyone crashes at the same time, gets upset easily because of sleep deprivation, or worse, needs to be hospitalized. Besides, it's very dangerous to stay awake for long periods of time since sleep deprivation carries with it more symptoms than almost any known disease. It's serious stuff messing around with your body's natural sleep cycles, so make sure you have someone reliable to take over for you so you can sleep.

Invite all of your friends to your house; the more people that come over the more interactivity you'll have. Fans of marathons like nothing more than new faces they can give nicknames to, especially women. It sounds dumb and sexist, but after seeing 3 or 4 men for 24 hours, a woman or charismatic new face that breezes in from outside for a few hours will reinvigorate everyone.

If you achieve your donation goal early try to send out a new round of e-mail to news sites to see if you can get a follow-up story, and extend the goal to encourage more donations and be sure to let the viewers know that they achieved that goal, not you.

Give the viewers more than you promised. If it's been a good show, they won't want to see it end and will demand an encore much like people do at a live concert. They'll appreciate the gesture of you playing an extra game or two, and will remember what a great time they had at your marathon the next time you do one.