Do it in small doses
You don't have to commit to an hour long spinning class in order to burn calories. If you can manage a 15 minute jog in the morning and then another one after dinner, that's 30 minutes of exercise. You can even do 10 minute bursts if that's easier. The point is to do what you can in the time you have.
Sneak it in
Any time you're moving, you're burning calories, so do anything you can to sneak in extra movement. Take the stairs, park in the furthest spot from the mall so you have to walk longer to get to the door, and when you're at the office, walk to the person's desk you want to chat with, rather than calling or emailing them. Every little bit counts!
Turn your next catch-up session with your girlfriends into a walking date. Rather than sit for hours in a restaurant or cafe, meet up and walk and talk. With all the chatting you'll be doing you won't even notice you're getting exercise. Alternatively, if you have friends who belong to the gym you go to, meet at the gym and take a class together. Sometimes all it takes to get to the gym is knowing someone will be there waiting for you.
Talking on the phone? Do squats while you chat. Watching TV? Do crunches or lunges during the commercials. You can easily squeeze in exercise while you're doing other, mundane chores like folding laundry or even washing dishes. You can even burn calories while you watch TV by doing jumping jacks, skipping rope or jogging on the spot.
Take exercise breaks
Whether you work at home or in an office, you need to take breaks in order to clear your head or just give your eyes a break from the computer screen. Rather than hitting the vending machine or just playing games on your iPhone, take a fitness break. Bring a set of dumbbells and a yoga mat to your office. Simple yoga poses or a few minutes of weight bearing exercise [bicep curls, shoulder presses] are a great way to get your blood moving and squeeze in some muscle toning on your break.