for me, the biggest motivation has been the people in this forum... and then the comments I started recieving from others who began to notice my weightloss... But above all, having people to support you and keep you going on the days you don't even want to get out of bed - much less go running - has been invaluable.
I also think that once you have started and get into a routine, that in itself is a motivator. After going two weeks straight of working out and eating healthy, I felt extremely bad about wanting to pig out one night. I didn't want to let myself down!!!
Oh! and don't discount the high that comes from exercise itself. I used to hate running, but now I love it, and I'm so excited to finally be back in a situation where I can do it every day again =) woot.