What sort of job do you do? I've managed to exercise at least once a week since the beginning of January, but I also think that a large part of my generally being fitter is that I walk around a lot more at work. Luckily the nature of my job is such that walking round almost makes me better at it! (Although my year 11 class said I made them dizzy one day...)
Do you travel by car or public transport? Can you get off a stop earlier and walk?
I'm a firm believer that anything you can do to make you more active in any way is a big winner.
I work at a desk and often work 12 hours a day. Then it's an hour on the bus home. The only exercise I get is the 30 minute walk to the bus stop and back each day. Most week days I get up, go to work, have something to eat then go to bed.
I know there's no excuse for not doing something at the weekend but a lot of time I just want to rest, meet friends or catch up on on housework after the working week.
It's a cycle I'll have to break because if I did get exercise I'd probably feel as if I had more energy.