In addition to everything else mentioned, I'd suggest bringing gloves and black hats and long johns to go under the sweats. Those are the things new NPCs tend to forget most often, it may be nice during the day, but at night it gets MUCH cooler, and sometimes you are waiting longer than you anticipate. I also suggest bringing multiple pairs of shoes and tons of socks, and a few changes of black clothes, in case of rain (which is far too likely, sadly).
Make sure your blanket/sleeping bag is very warm (I've made the mistake of bringing a 40 degree bag to an event with 20-30 degree nights, it's miserable, and it usually results in post-nero plague)... Depending on your site you may want an air-mattress (I'm not familiar with the MN site at all)...
Once you are there, it is VERY easy to forget yourself, try not to. Make sure you keep hydrated (even if it's cold), you are using alot more energy than you think and you are using up a lot of fluids. This will sound silly, but don't forget to eat, make sure to bring some snacks for yourself (in case) like granola bars or trail mix but try to keep it less salty since that will cause you to get dehydrated too easily, food is often provided for NPCs but it's good to have your own in case. It is VERY easy to get busy and forget you haven't eaten in 8 or 9 hours. If you are tired or need a break, don't be afraid to say it, also make sure you give yourself some down time, get some sleep and don't keep running -- that's easy to forget too your first weekend (believe me the adreniline kicks in and you can go all night, but you WILL regret it on Monday!)
John's (through Scott) & Eric's statements (through Jena) are 100% right on, you are there to entertain the PC's... don't forget that, if you aren't sure of something, err on the side of the PC. NPCs get to be the world for the PCs they are everything from the monsters to the peasants to the tavern help to the tree moss (don't ask), make it a challenge for them, make it interesting for them but most of all make it FUN for them. I think Scott said this, but I may be wrong "Ask yourself constantly, if I were a PC, would this be fun for me?".
My 2cp,
~Kelly