Even if daily exercise is already a part of your life, there are bound to be times when your motivation flags. It's important to have strategies in place to push through those tough times.
Personally, I find it helpful to remember that the many benefits of exercise -- increased energy, reduced stress, etc. -- come after I've finished working out. When the alarm goes off early in the morning, of course it would feel good (at least temporarily) to go back to sleep and skip my workout. But later in the day, I know I'll regret it. Instead, I remind myself that in exchange for 45 minutes of exercise, I'll feel better all day long.