Unfortunately, staying physical fit does not come easily. It takes a great deal of determination, planning, and motivation. Peeling oneself off the sofa and into the gym or out for a run requires a great deal of motivation.
Establishing goals is a great way to find self motivation. One does not always require goals to find motivation though, sometimes the use of logic can also prevail. For example, you've had a very long day and the last thing you want to do is exercise. The easiest thing would be to sit on the couch and veg out. Think about this though, in the amount of time it would take to watch a 30 minute television show, you could go for a walk outside or if you live close enough, hit up the gym for a quick 20 minute workout. Odds are once you get to the gym, you'll want to stay longer anyway.
Another great way is to find an exercise partner. It is much more difficult letting someone else down than just letting yourself down. Utilizing an exercise partner forces you to be on time and prevents you from making excuses for not exercising. You don't want to leave your partner hanging do you?