PFunk said:
Planning helps. If you know what you're eating and when, and therefore don't have to make any decisions, you can't choose to eat badly.
Same applies to the gym.
Set in stone: Monday, Tuesday, Thursday, Friday after work I go to the gym. No Decisions to make.
No getting to the end of the day and deciding you don't feel like going to the gym.
Tip: You have to be genuinely unwell or exhausted to not go to the gym. There are no other excuses. Plus you'll feel heaps better afterwards.