I think you need to identify what motivates you. As with many people here, numbers, and specific goals are very motivating to me.
I love cardio machines that tell me how many calories I am burning per minute, the distance, allow me to check my heart rate without actually doing math and can see that I am in-the-zone at a glance so to speak. I have learned to turn the things i CAN'T do into motivations. I will, I swear someday be able to get up from downward dog properly instead of having to widen my stance, I will continue to strive to do the Plank for prolonged periods of time. I make these little goals for myself and congratulate myself when I get closer to accomplishing them.
I am a introvert but a people person, for me going to aerobics and yoga classes is not only a physical thing it is VERY much a social thing, afew of my friends and coworkers go, we all encourage each other to go, we talk about it before, and after it is a common bond. I am a person that thrives on common bonds even though I also like to sit at home watching DVD's. Also I set a weekly date with a friend to workout one day a weekend at the gym its quality time we spend together, we crank up tunes we talk as we walk to and from the gym. Its something positive in our lives to do together and that is a motivator to me.
I love to dance, to groove, to move to music, so whether it is that my aerobics instructor has impecable taste in music, or that I pick songs that make my heart soar when I'm doing cardio by myself activities where there are music motivates me.
I love basketball, i'm not so much of a runner but I'll run to try and play defense even though I probably shouldn't. I loved playing in high school and I'm trying to casually get back into it now. Again thats also a social thing for me.
I'm also a day dreamer so walking ? If i'm in a warm comfy place whether its outside in the winter or on a treadmill or a indoor track in the winter, I can daydream guilt free, I mean I'm excercising so its not like its totally self indulgent time
So I guess its taking things you love, and doing them, OR pairing up the things you love with the excercise. The you associate the excercise with the things/people that you love and.. it becomes something that you love as well.
At least thats how it is for me, I'm hardly what you'd call super active (aerobics 3x a week, yoga once a week, gym once a week, and sometimes a game of basketball...) but that activity level is getting comfortable for me
I'm concious to make sure that I appreciate that I am LUCKY to be able to do all that. And I phased it all in gradually, first the aerobics, then the yoga, the the basketball and now 1 gym day a weekend