Discipline means going even when you may not feel like it because you know it will pay off in the end.

Set a goal for yourself - # of pull ups, # of miles on a treadmill or hitting a certain weight in the squat.

Set a reasonable # of times to go to the gym even if it's 2 times a week... pick a number where you can ensure you're consistent.

Get a good workout program so it's actual training (working towards a goal with an end result) and not just "working out."