Unless you have a chiropractor on speed dial, you need take some practical steps to keep your bones aligned. Before you invest in a state of the art ergonomic wonderchair, consider that the same solution, as expensive and cutting edge as it may be, may not work the same way for everyone, and no ergonomic chair will work as promised if you don't sit up straight. The bottom line is that you have to find a position that's comfortable and practical for you, and it doesn't always involve a wonderchair (but sometimes it does).
One of the keys to eliminating soreness during a long day is simple: move. If you sit in the same position without moving, your body locks in position and the result can be body aches and pains. Web development does not have to be bad for your health. Take regular breaks, stretch, and walk around a bit. A good natural reminder is to constantly sip a big glass of water. Your body will force you to go to the bathroom several times a day – all you have to remember is to stretch when you get up and refill your water.
The ideal comfort position for most people is sitting straight up but relaxed, with your head just slightly forward. Your monitor should be directly in front of you, not at an angle, with the top of the monitor at eye level so you don't have to tilt your head back to see it. The seat of your chair should be adjusted so that both feet are comfortably on the floor and your thighs are level. This usually means taking your keyboard off your lap and your feet off the desk.
If you start to experience achiness in your calves and thighs, elevate your feet a few inches. Telephone books make great elevators, but for a real treat, put a vibrating foot massager under your desk. Mine even has heat for those chilly mornings.
Some we developers prefer to stand as they work. If you decide to stand, invest in a good pair of shoes and a footstool, and place your workstation at the proper height for your arms to be at right angles to your body.