I did a my first trail marathon about a year and half ago, great experience and training...elevation was definitely tiresome, going from 100 feet to almost 2000 feet, and then repeating itself.
For Training try this website - it helped a lot in the schedule.
Hydration - either invest in a good camel back pack, but use more of the refueling stations to get you going. Also, carry some needed fuel with you so you also get accustomed eating while you are in mid stream run or close to a station. I do not like the goo gels, they are to nasty to eat in the sense of the immediate sugar rush you get.
Weightlifting and Core Workout essential - ensure that you incorporate weigh lifting and core workout remember that you will be running for an extended period of time, ensure that you have a strong abdominal area for support, it will get tiresome, but you want your core to be strong to carry you through...the worst feeling is if your legs still have energy but you start to have back pain or just muscle fatigue in your upper chest and shoulder areas.
Mental Stress - be prepared that at some given point your mind will want to stop...they call this breaking the wall sort of speak...in essence your mind will want to stop, it happens to all runners. It can occur at any given mileage, therefore train for long run during your weekends, and your short distance runs during the week. For me it was at mileage 22 that I hit the wall, and just had to push myself to continue going.
Clothing- what everyone has already stated..nothing new the day of the race, for quick gear cold weather/warm change get some running sleeves...such as http://www.moeben.com/, this can easily be put on if its too cold or taken and store away while you are running.
After the race, you will be very soared ensure you continue to hydrate properly through out the week. Try to soak your muscle in Epson salt or use of a hot tub, preferably with a hot man (lol). If you can get a compression outfit that will help with muscle recovery, even better. Also, you will be extremely hungry which is normal, just pace yourself. On my given race per my heart monitor I burned 3800 calories.
Lastly, enjoy and tell us how you did...best of luck.