Exercise in an important part of life, since it is the way the body grows and adapts to new challenges. Without exercise, the body grows weak since it is not being forced to adapt, and weakening strength in the body also weakens immune responses and healing.
One of the best ways to find time to exercise is to incorporate it into the daily schedule as a part of the actual work. For example, if you live close enough to your workplace, you can start walking to work instead of commuting by car or bus. While this will require determination in waking up earlier and walking through uncomfortable conditions, a daily walk will help prevent heart disease and strengthen the entire body. This is an excellent gateway into longer and more deliberate exercise, but even on days when you are extremely busy, you will get at least the benefits of the walk.
Another way is to learn small or short exercises and practice them during lulls or downtime. While on the phone with a client, for example, stand up and do a few knee-bends or simple stretches. Keep a pair of five-pound dumbbells by your desk and do curls and shoulder lifts while a document is printing. Yoga poses are low-impact, great for stretching, and there are many books and websites with ten-minute yoga routines.
Of course, you can also try to get to the gym on your lunch hour, but that often limits you to a 30-minute workout at best, counting travel and dressing time. Any exercise is better than none, but it is often more beneficial to sneak in short workouts during the workday than to skip lunch -- which is vital for good health -- to perform an exhausting full workout. Remember, any activity is better than none at all.