Just lie to yourself, every so often. Everyone has those low energy days, which followed a night of poor sleep or a bad day at the office, makes it understandable that your daily workout feels like an unnecessary encumbrance on the larger, more ominous obligations to your day. Prepare for your workout by doing your normal warm up, but figure on doing 1/3 of the total workout you normally do. Tell yourself that your energy is low and once you've stretched and done a modicum of aerobic exercise, you'll go home. Your mind will appreciate the insincere sincerity and your body, once you've circulated your blood a few times, will be ready as ever to get the full workout going. Sure, it's a crap shoot, some days, you may actually be too tired to do a full workout, but the chances that you'll finish 100% of your goal is better than you may think.