I use a standing desk. When I first started out, I spent a lot of time thinking about how I could make a standing desk that I could raise and lower. I looked at taking apart office chairs and using the hydraulics, I looked at possible crank methods, weight and pulley systems, etc.
A friend of mine listened to my brainstorming and said "why don't you just get a taller chair?". He was right.
This $140 drafting chair is what I have now. It has a padded seat, back, armrests, and is just as comfy as any other office chair I've tried. http://www.amazon.com/Boss-Multi-Function-Leatherplus-Drafti...
There are cheaper ones that are more like a stool, more expensive ones, and even some weird (but expensive) saddle type chairs that are designed to mimic standing while you're sitting down. But basically, if you want a standing desk and the ability to sit, ie if you're going to be buying a chair anyways, just buy a taller chair.
It takes a week or two for your feet to get used to it, and if you slouche or lean, you'll find yourself with sore hips at the end of the day. But once you get used to it, you'll find yourself standing much more than sitting. For me, I mostly only sit if I'm eating at my desk, or maybe for a bit after lunch. I have a standing desk at work and one at home. It's a different experience just walking up to your computer and start typing. Most people have to sit down first or stand up when they walk away. This lends to a sort of reluctance to switch from one state to another. Standing, I can lock my screen and walk away at any time.