I work out early, too. I get up at about 4:45 most days to get animals fed and a little coffee and something to eat before I head out. It's hard.
It's easier for me when I get everything ready the night before...clothes, gear, check the morning weather forecast. I think someone up-thread mentioned this already.
But, the other thing I do that helps *a lot* is, right before lights out "remind" my husband that I'm working out in the morning. He doesn't need the reminding -- I do it because it helps cement the idea in my mind. And then, when I'm falling asleep, I sort of visualize myself getting up early, putting my workout clothes on, having my cup of coffee, and heading out the door for my run or bike ride.
It sounds hokey, but the visualizing *really* helps me actually get out of bed the next morning.