Seeing as how layout marathons are becoming much like an epidemic here at the TAFL boards, here are some guidelines if you want to run your own =)
1. Post a new topic for each marathon you're thinking of running, in the
Layout Marathon subforum.
2. Decide on a particular subject for the layouts for focus on. It could be anywhere from characters from 1 series, characters from series by one mangaka, bishounen/bishoujo characters, etc. Let your imagination run wild! But keep in mind that the broader your subject is, the more people might join the marathon =) Just a suggestion: try not to pick a subject that has been done recently. Otherwise, you might not get as many participants.
3. Check out whether there are other marathons currently running. When deciding when to run your marathon, make it so that it doesn't coincide with other marathons. Also, choose a comfortable time period to run the marathon. The usual is one month. If you are thinking of running a marathon concurrent with another one, try asking the owner of the other marathon. Also, mention that skinning is allowed in your rules so that participants can be in both marathons.
4. When posting about your marathon, have the few key facts in the first post:
Subject: the focus of the marathon, as described in guideline #2.
Date start: when all the layouts must be up by
Date end: the earliest date that all the layouts must stay up by, usually one month.
How to join: either have a separate join form or have people post in the thread
Who can join: domains, blogs, etc.
Any specific rules: basically your own rules, such as "one layout per characters/series".
5. Have some kind of linking system. If your marathon goes over 20 members, a button rotation is suggested since 20 buttons on a page might get tedious. But it's really up to the owner. If you're just using linking buttons, specify in your first post whether the buttons must be made by the participants or the owner of the marathon. Also, it's a good idea to jot down everyone's email so you can contact them easily.
6. You should update the first post with the participants list.
7. It might be nice if you have a FAQs page if your marathon starts getting large or if people start asking the same questions.
Again, these are just guidelines. Good luck and have fun marathoning!