Daily workouts. Sometimes there's no time (or motivation?) for the usual routine. Then you're left with the nagging feeling of needing to do something. Consider a "when all else fails" workout.
Julie Deardorff of the blog Julie's Health Club taps exercise specialist Phil Black to explain how to create a workout that's not dependent on anything but your own steam. Focus on a favorite body-weight exercise, Black says, and repeat. For him, that means doing 100 pull-ups or 100 push-ups. (No, not everyone should do that many.)
"But for those of us who consider one pull-up a challenge, Black recommends substituting his third favorite full-body exercise: squats," Deardorff writes in her blog. "Just plain old regular up-and-down squats," Black told me. "No weights or bars, just up and down with good form."
Here's something else that can help. This transcript of a recent Web chat with trainer Mike Donavanik lists some resistance exercises you can do at home to get a good workout.
All of this sounds pretty simple -- provided you actually do it.