Have a plan and a goal in mind.
For instance, running, I've always had a race that I've been training for, and after trying really hard to just run (can't see how people do that without anything) I've started making time goals and such to keep my mind and body challenged. It's all about having a goal...your body will not react if you just want to do something, at least, it won't be easy to make it a habit.
Secondly, you need a plan. When I train for races or just for PR's, I need distances, times, and such. Then I write it out all on my calender. You probably need to find some kind of running plan (or whatever exercise) plan online.
Thirdly, it needs to be at the same time of the day.
If you have any questions about running, feel free to PM me! 
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Go die in a ditch if you're a b*tch, if you're a jerk, go to work, if you're just mean, flee the scene, and if you're rude, go ahead and intrude because you're probably just like me.