This is a question I get asked all too often, and I completely understand the feeling. While your initial reaction to feeling pooped at the end of your day might be to skip your workout and head home, I wouldn't recommend it. Unless you're sick or you've been over training, a lower intensity workout is better than nothing. Something is always better than nothing.
Depending on what my workout was 1-2 days before, I often feel stiff and sore, which isn’t the most encouraging feeling when I’m going into another workout. It isn’t until I get going in my warm up that I start feeling good. That being said, start by just getting yourself to the gym. That alone can be enough to get you amped for your workout.
Next, do a dynamic warm up. Get your blood flowing. Roll out, dig in with that lacrosse ball, stretch, brisk walk on the TM, corrective exercises, etc. If you feel up to your full workout once you're warm, go for it. If not...
Bring down the intensity.
If you’ve warmed up and you’re still feeling rundown, cut out any explosive moves (plyometics or Olympic lifts). If that’s not enough, dial down the intensity of the rest of your workout by decreasing sets, reps, and weight, and increasing rest time between sets.
Make it earlier.
Having been in the fitness industry for over 10 years now, I can tell you that most people perform significantly better in the morning, or mid-day, before they’ve killed themselves all day at work. The day's stress hasn't come into play yet, so you’ll be able to give everything you’ve got to your workout, which will increase the speed at which you see performance gains and body composition changes.
Lastly, if you are going to workout after work, you have to make it a priority. Put your workout in your calendar as an appointment, bring your gym clothes to the office with you so you can change there, and leave with the mindset of going to workout. If weekdays really are a struggle for you, shoot for 2 workouts during the week and get 2 more in on the weekends. Make sure you’re taking periodic breaks during your day to get up, away from your desk, and stretch or go for a little office walk, stay hydrated (goal is at least 2L H2O per day), and eat healthy protein based snacks. These things will help keep your energy up throughout the day, and increase your chance of making it to the gym after work.
Bottom line, If you want the results, you have to put in the work. But I always want you to be kind to yourself; if you really need the rest, take it.