I have chronic back problems (lumbar) and I've tried dozens of chairs over the past few years from periods between a few hours and a few weeks and I've only found 2 which haven't caused me problems:

Herman Miller Aeron (which I currently use) Steelcase Leap Char (which I used at my previous 2 jobs)

The price for these chairs are high. But given I'm relatively healthy and productive with them, its well worth it.

At work I have a Leap. It's pretty good (lots of knobs to adjust), but at home I use the Herman Miller Embody. Personally I feel it's more comfortable than the Leap, but chairs are very subjective.

You may like it because the seat "pan" isn't actually moving the seat. It extends the front of the seat further out. That way you don't feel like you're sitting off the back end of the seat.

Have you had a chair fitted, or read the instructions on fitting? If you haven't I highly recommend it. The Steelcase vendor was in my work the other day explaining proper posture on the chairs. You're not supposed to sit up, but rather recline at a slight angle.

I am 6'4", and I recently bought a Zody, review here: http://www.geek.com/articles/gadgets/review-haworth-zody-cha...

I actually prefer this to the Aeron chair that I use at my office. I like it so much that I bought a second one for my girlfriend.

I'm 6"2. I've used the Leap with not too much success—too short in the seat pan, and the Amia which actually worked fairly well for a while, despite having the same length seat pan (19"). I'd really like to try the Aeron, but no one in town deals them.

Unfortunately I often have problems with my lower back. Thing that helped me the most was plain exercise (pilates) ball. It forces you to sit correctly and keep your back straight. It can be hard at the beginning but when you get used to it :)

I have one of these: http://igoergo.com/ (I think it's an older model not listed on the site.) They're not beautiful but are very customizable including bigger or smaller seats.

I have a chronic neck issue. I've tried a bunch of high end chairs, and had the best luck with a Steelcase Think. It's very possibly too small for me (I'm 6'1"), but everything else I've tried flairs my neck in bad ways.

Humanscale Freedom Task Chair: http://humanscale.com/products/product_detail.cfm?group=Free...

I had a lot of problems with chairs, I couldn't seem to find the right one. That is when I realized there wouldn't be a right chair. I converted to a standing desk, haven't looked back since.

I'm 6'4" and have dealt with posture issues. Lately, I've found that the low-back Eames management chair in combination with a forced effort to maintain good posture yields pretty good results(takes a while to get used to but it's worth it in the long run).