I stay motivated through using variety in my workouts. I love doing anything outside (running, hiking, biking, swimming, snowboarding etc.)
I have found working out with other trainers, or having another trainer train me, to be particularly beneficial. Someone else can usually push you harder than you can push yourself. It is also nice to not have to be the one to plan the workout for a change. I usually learn new ideas to use on clients as well.
Signing up for different races also helps keep me motivated. I recently completed my first triathlon which forced my to add a lot more swimming and biking to my workouts. I try to challenge myself the same ways I do my clients. (If I suggest to a client sign up and train for something, I feel I should be challenging myself in the same way).