Some days you're just not going to be feeling it. Flopping on the couch and staring at the TV may be much more appealing. The thought of an entire workout may seem overwhelming; after all, you're already mentally taxed and physically tired!
On days like this, just get to the gym. Don't worry about what you're going to do, what weight to use, or how long it's going to take you. The hardest part by far will be just showing up at the gym. When I feel this way I tell myself to just get on the gym floor and train for a few minutes — maybe only a few sets of a few exercises. The first few reps may seem brutal, I'm just not in the mood. I just take it one rep at a time. One, two, three, four... the next thing I know set one is done, then set two!
Wow! Now I'm starting to feel it! Blood is flowing, endorphins are being released, I'm feeling a sense of accomplishment, and muscles are burning! I look in the mirror and see the muscles getting pumped and veins popping out! Oh Yeah! Now I'm happy that I came to the gym! I just had to take it one rep at a time! You've got this.