My best tip for motivation to exercise is to do it first thing in the morning on an empty stomache. First off, you get it out of the way so that you don't have to worry about it at the end of the day. I know how it is to get off work and not want to do a single thing besides lay around the house. Second, if you work out in the morning before you eat anything it burns pure fat (not calories you've stored up during the day) and boosts your metabolism. You're already doing great though so I wouldn't worry too much. :-)