If you chose the Maffetone Method in the “Training Plans!!!” section, you'll become intimately familiar with heart rate monitors. Even if you didn't, a heart rate monitor could be a valuable tool. Not only is it cool to see your heart rate in real time, it can be used as a great training tool to prepare your body for ultras.
The idea goes something like this if you run slow enough, your body will burn primarily fat instead of carbohydrates. Since most of us have well over 100,000 “fat” calories stored in our body and only a few thousand “carb” calories, it makes sense to burn the fat. Besides that, the “bonk” or “wall” marathoners complain about it caused by your bodily supply of carbohydrates running low.
It's actually really easy to train your body to burn fat. I do two things:
First, run your long runs slow. This is where a heart rate monitor comes into play. If you keep your heart rate low (here's Maffetone's formula: http://philmaffetone.com/180formula.cfm) on your long runs, you'll train your body to better utilize fat stores. There are other possible positive benefits, but this is a biggie.
Second, do at least a few of your runs after fasting. Don't eat for 12-24 hours prior to the run. You will reach the “wall” much faster as your body's supply of carbohydrates will be much lower. This isn't nearly as effective as the first technique, but it does familiarize you to the feelings associated with hitting that wall. If you experience the beginnings of those same feelings in a race, eating something sugary will usually reverse the effects. Knowing your body and the signals it's sending is important.