If you’re trying to build new habits (or just desperately trying to hang on to some of the ones you’ve been trying), check out Gretchen Rubin (of The Happiness Project fame) advice.  First you need to understand your personality type when it comes to the rules you set for yourself and the rules others set for you. Which personality are you

Upholder—accepts rules, whether from outside or inside. An upholder meets deadlines, follows doctor’s order, keeps a New Year’s resolution.

Questioner—questions rules and accepts them only if they make sense. They may choose to follow rules, or not, according to their judgment.

Rebel—flouts rules, from outside or inside. They resist control. Give a rebel a rule, and the rebel will want to do the very opposite thing.

Obliger—accepts outside rules, but doesn’t like to adopt self-imposed rules.

So with that in mind, an example of  how you can get your exercise in:

An upholder can train with a trainer or exercise on her own; a questioner can do either if he thinks it makes sense; a rebel will do neither, because the fact that she has an appointment or an item on her to-do list makes her want to disobey; an obliger can meet a trainer, but can’t get to the gym on his own.