While your initial reaction to feeling rundown may be to skip your workout, don't avoid the gym. Unless you're sick or you've been overtraining, a truncated workout is better than nothing.
Start your workout with dynamic stretching. (Click here for a sample routine). During your dynamic warm-up, you may start to feel more energized. If you feel up for a full workout at this point, go for it. If not, cut out any explosive or technique-intensive moves and dial down the intensity of your normal workout by decreasing sets, reps and weight.
A great way to avoid letting fatigue sidetrack your workout is to hit the gym in the morning. The day's workload and stress hasn't come into play yet, so you'll be able to give your all in the gym. If an afternoon or evening workout is your only option, take periodic breaks from your desk, stay hydrated, and eat healthy snacks every 2 to 3 hours to keep your energy up throughout the day and increase your chance of making it to the gym.
-- Kevin Elsey