There are some very good tips here already, but here's my tuppence.
Enjoy It Find an activity you enjoy, it could just be walk the dog, cycle round the park or go for a swim. If you enjoy it, you are less likely to switch off and see it as a 'task'.
Go Steady You do not have to be blowing out of every orifice or soaking with sweat to reap the benefits. Moderate exercise for more than 20 minutes a session three times a week is superb for making you feel good (it helps to combat depression) and keeps you supple. Try doing sessions on alternate days to give your body time to rest in between.
Warm Up & Cool Down Warm up by gently easing into the workout of your choice, dont just go from rest to full tilt as it wreaks havoc on joints and muscles. Cool down by stretching off afterwards, make sure that you stretch all your worked muscles, it helps to avoid cramps and muscle fatigue after.
Do Not Be Embarrassed Do not feel embarrassed about what you do or how you do it. I used to get put off in the gym as being a fat soldier surrounded by green lean drinking machines, I was shy of doing my thing - I had to work out my mind first and programme it to think 'balls to them'.
Do Not Punish Yourself Last and by no means least. Do not beat yourself up mentally if you miss a session - this is fatal and demotivates you more than anything. You would not beat yourself up if you forgot a household chore, so dont worry about missing a session, just programme it for another day.
Chin up and keep smiling
