If you work late, turn off any fluorescent lights, which are high in blue hues—the wavelength to which our time-of-day sensors are most sensitive—and use an incandescent lamp (higher in reds). If you're at a computer, lower its brightness setting.
Banish the TV from the bedroom (you can always DVR The Daily Show). Reading in dim but comfortable light before sleep is less disruptive to your system than the light from the tube.
If you need to get up in the middle of the night, try not to flip any switches. (One burst of brightness can temporarily suppress your body's production of melatonin.) Instead, keep a small flashlight by your bed or a night-light in your bathroom.
Make sure you eat breakfast: It helps signal your body that a new day is beginning. And catch some natural rays from the sun—perhaps by walking part of the way to work.
If you're indoors most of the time, try boosting your sunlight exposure by sitting near a window or eating lunch outdoors.