First I would just say, plan on things taking longer than you think they will. If you think you'll need an hour for registration... be there for 3.
Second, I'd call the insurance or if you're not the person directly insured call whoever handles the insurance and make sure they will specifically cover tournaments. Some will cover general training but require seperate insurance for an event. Make sure you know exactly what their requirements are as far as protective gear for participants, how many first aid personell they may require you to have on hand and what level of training they require them to have. Make sure the protective gear requirements and all rules regarding contact are clearly defined in the registration forms: all disallowed targets, what level of contact is allowed (light,med,full and define that verbally when you address all contestants at the beginning), what protective gear is required for all contestants.
We usually line everyone who is competing up at the beginning and give a demonstration of all the rules, etc. so there is less chance of a competitor saying they thought this or that was allowed or that they didn't know they had to have a mouthpiece and cup. Clearly demonstrate the level of contact that will be acceptable and what will not be. Enforce these with an iron hand if you don't want to run that chance of dealing with a lawsuit. You may anyway, but at least you did everything in your power to clearly define the rules and expectations.
Prior to starting any actual matches personally gather ALL judges/referrees/time keepers/etc. Give them all specific instructions on how to run their areas and make sure they know you are serious about this. No ambiguity here. If everyone must have headgear you make sure everyone knows that fact and will face consequences from you if they don't enforce that in their rings.
Try to get a mix of judges in each ring from the various schools so this or that ring isn't all judged by one school which may lead to cries of, "I was robbed". Explain to all contestants in the aforementioned line up and demonstration that judging is hard, that you yourself have been robbed plenty of times, and they probably will too this day. It's nothing to start a riot over. With regards to your judges I usually instruct mine that if they're reffing a match between one of their own students and another schools, they should intentionally be overly picky with their calls on their own students. If I'm center judge in a fight between my best guy and another school's... he better leave no doubt becuase anything remotely close will be called against him. I think you get the point.
Just as I said to start early, plan for things to run longer than you expect. Don't rush things. Don't put any of the contestants in danger or make them compete in a rushed and poorly put together ring becuase you're trying to keep schedule. Try to keep schedule, but it's more important to be safe and in control of everything. Because, if anything goes wrong, it will come back to you. So don't fear calling a lunch break or asking for peoples patience so you can finish category Y before setting up to do category Z.
Scared yet? Don't be. Have fun, but just be in control. Just as I assume you're in control of your classes and students. It will work out. But these things do take practice before they start to go off smoothly.
Good luck. Have fun.
edit *Pick out some of your top students/instructors and delegate these jobs. Don't try to take on everything the day of. Be the pit boss. Roam from ring to ring and just ensure your assigned people are doing what they're supposed to do. If they're not, pull them aside and reinstruct them. Then move on. Don't get tunnel vision. As the general host you need to see the big picture not concentrate on the tiniest details. DELEGATE...but delegate to trustworthy poeple once again.