I know, I know. Diet and exercise is the nauseatingly repetitive advice for best warding off many health problems. And unlike claims by some enthusiasts about the "high" you enjoy while exercising, the best part of my workout by far is when it is all over for that day.
Exercise for me has been purely an obligatory part of physical maintenance, and I used to follow a home regimen some days a week. So if you've been having problems overcoming inertia or a hectic schedule I'll encourage you to be more active. But trust me, I feel your pain.
Fifteen years ago my (then future) sister-in-law Deanne first helped overcome my reluctance to enter a gym. She introduced me to some exercise equipment and I've been a fairly regular gym goer since then. I'm glad it's paid off, but I've had sporadic problems because of wrong exercises or poor form that I learned about and corrected only subsequently.
But today I happened upon this excellent article with illustrations on WebMD.com about nine bad or least effective exercises, and better ones in their place. I'm guilty of at least three of those nine lapses, and wish I had seen this article earlier. It's certainly worth a look.
For those who don't like the concept of gyms or using exercise equipment there's also this useful article about seven "most effective" exercises that can be done at home. Five of these require no equipment at all and in the other two dumbbells or simple home objects can substitute for the barbells shown. Of course there are several lists of "best exercises" featured in magazines and other publications, and this is just one of them.
Finally, here's the Wikipedia description of the classic 5BX / 10BX system for men and women (with a link to download it) that takes just 10 - 15 minutes a day. I used this for many years after it was taught early in my service career, and still like to go back to it from time to time.