I prefer working out in the mornings, as it is too easy to get sidetracked later in the day by other obligations.

It helps to get the workout clothes ready the night before, and put them where you see them first thing in the morning. I was up at 4:30 AM this morning, and doing a weight workout by 4:40 AM. It gives me a sense of accomplishment for the day, and helps me to wake up and feel more alert and ready to take on whatever comes my way.