DeviantART alone has a lot of reference material you could use. In the search bar, type (for example) "sitting chair". Then on the left side of the category click Resources & Stock Images>Stock Images>Model. There should be a large quantity of photos for you to draw from.
Also, basic shape and drawing a light blueprint of your sketch helps immensely. Rough out the shape as lightly as possible and slowly build up towards the finish pose you want. It's a bit easier said than done, but studying anatomy and the way the human body works and torques will help your figures look more natural. I myself still have a long way to go, but doing the occasional figure study helps me get better.
Good! Get a chair, set the camera timer, and take a picture of yourself in a chair. Then use the pic as reference! This will be hard at first, but it gets easier.
Er, I guess also you could have a patient friend sit in the chair while you sketch. That never occurs to me, because I don't know anyone who sits still for that long. OR you could use a big mirror, but that's kind of tough because you're looking and drawing at the same time.