Results! When you see yourself getting bigger and stronger, or more cut depending on your goals, that's enough motivation to get to the gym even if you're tired. When people start noticing and complimenting you on how you look and the girls you date get progressively better looking LOL and you feel more confident about yourself, you're not going to let being tired stop you from working out! Plus like Cordoba said, once you get there and pump out a few sets, you'll feel great and want to finish.