Life happens, so you can't leave your workouts up to chance and when you can fit them in. This is no different than fitting other important things in to your day. Everyone has different schedules, so I suggest you find a time that works for you. For me, it's easier to wake up insanely early. Even though 4 am can be painfully early at times, I enjoy the quiet in the house. There's something's peaceful about being up when the rest of the of the city is still asleep and I'm working on my fitness. If I wait until after work, then I'm often too tired and I'll get distracted by my to do list.
I schedule most of my workouts in the Super Gym on the Beach Body website. It's free and helps me stay accountable. Also, when I follow a program it keeps me from getting I a rut of choosing the same workout. The schedule is done for me, so that helps keep my muscles guessing and burning calories.