I've found that it's easy to make a good dent in my daily walking goal by spending half of my hour-long lunch break outside. 30 minutes of walking purposefully (and not really even breaking a sweat) can make all the difference. I return to work refreshed and energized, having taken in some fresh air and gotten my blood flowing. It's also a great opportunity to think through things and aleviate stress!
I try to pick restaurants, grocery stores, and other shopping within walking distance so that I can get some exercise while doing errands. I also try to listen to podcasts while I walk or do errands.
I can catch up on tech news, get some exercise & finish errands at the same time. Multi-tasking FTW :)
I second (third?) what you guys said about the lunch walk (I find it also clears my head!), the errands, etc.
I also sometimes listen to lectures that I get from The Teaching Company (teach12.com). Learning history while wandering is nice.
Also, one thing that keeps me motivated is watching my neighborhood change throughout the seasons and the year. Soon it will be February and the daphne will bloom. It smells so good!
Also important for me is that I carry a mini-umbrella in my bag at all times. Getting deeply moistened is a buzzkill.