Sitting at the computer all day is not exactly good for the body.
If you have to be at a desk all day long, doing some simple things can improve your posture and health.
1.- Sit properly in a good chair designed for desk work. Your back should be straight, your shoulders back, and the top of your monitor should be level with your eyes. If you have to look down or up, you need to adjust the height of your screen. Also, make sure that your wrists do not lay on the keyboard or on the mousepad (unless you have a pad with a wrist rest). This will help prevent Carpal Tunnel Syndrome. Keep your legs bent at the knees so that the knees are only slightly higher than your hips. Feet should be flat on the floor or on a step stool of some sort.
2.- Stand up every half hour to stretch or walk around a bit.
3.- Stretch your calves, and give your eyes a break from focusing on your computer screen. This will also help prevent blood clots from developing in your legs. Blood clots are very common among middle-aged computer users.
4.- Learn to stretch. To stretch your neck, flex your head forward/backward, side to side and look right and left. Never roll your head around your neck. This could cause damage to the joints of the neck.
5.- Roll your wrists regularly (this will help prevent carpal tunnel syndrome if you spend a lot of time typing).
6.- Roll your ankles regularly, this helps improve blood circulation (and that tingling feeling you can get when blood circulation is cut off, also known as "Pins and Needles").