I find that just having a set training schedule to follow is what I need most to stay motivated. Im a very goal-oriented person and as soon as I decide on doing something, I find or develop a plan for achieving it and then post it in a place that Ill see often every day. Fear of failure and a training log keep me honest and training hard. When Im not out running, I stay motivated by reading books and magazines, visiting websites, and participating on online forums such as this one.
Others I know have found it very helpful to run with a marathon training group, but I dont have any personal experience doing so.