The hardest thing about doing a 100 mile trail run is that it is 100 miles long. There are NO easy courses, sure fire training schedules, miracle foods or magic potions. Looking back to what was the single most important factor in my completing my first hundred miler, I believe it was obsession. Reading in Ultra magazine about 100 mile trail runs and that only 1000 or so runners a year accomplish this feat fascinated me. Knowing that failure was a real possibility made the challenge that much more appealing. Deep down inside I knew I had to do this. At night I would dream about it. To me it was as much a mental challenge as a physical one. My philosophy was the best way to learn to do these was by trial and error. Whenever I stood at the starting line of an ultra from my first one until today I evaluated my physical condition, training, weather, course conditions and tried to decide what I would have to do that would give me the best chance of surviving this one while experiencing as little discomfort as possible. If I would fail in any way it would be a learning experience that would help me next time.
    I believe experience is the second most important factor that will increase your chances of finishing a hundred. Experience allows you to be self-reliant on the course. To me being truly self-reliant means going to a 50 or 100 mile race without a crew or pacer and feeling confident you will be able to get by with the race provisions and your own preparation. Any help you get from the outside is then  just a bonus. You have to learn how to work through things, learning to deal with all the emotions and physical rebelling your mind and body may surprise you with will be uniquely valuable to you. At times during the run you will feel like the strongest person in the world and at others a feeble weakling. Your emotions will range from sheer joy of being to hating yourself for being so stupid to put yourself through such  an ordeal.
    One tip I would give you would be to decide far in advance of the event under what circumstances you would drop out of the race. Do this with a clear mind because after you are out on the course 15-20 hours you will not have the same brain you started with. If you do drop, you don't want to realize a couple of days later you probably should have pushed on. These thoughts will haunt you until you have a chance to vindicate yourself by starting over again at mile 0. IT DOES NOT ALWAYS GET WORSE. If your brain does start telling you to call it a day always try to go one more aid station beyond where you wanted to quit and re-evaluate. If you still feel like quitting, sit at that aid station for a while and watch the runners coming behind you. You will see people who look as bad or worse than you do but somehow they push themselves onward. They still have the "obsession" and there may still be time to find yours.
    The emotional high of finishing will last a lifetime. The day there is not a tear in my eye when I cross the finish line of a hundred mile trail run is the day I will stop doing them. There is a bond between all 100 mile veterans, even if it is rarely verbally expressed. We have all seen the same demons and have had the obsession on at least that one day to overcome them .