I agree with Dennis445 and others: more density to the mesh at the joint and changing the way it influences the mesh with the weight thing. I like to make joints like elbows, knees look like the mesh is wide on the non bending side and narrow where it bends... like the knee is a triangle with the base being the knee cap and narrowing to the crease. Then when it bends the knee mesh will already be shaped for the bend. Another thing you might do is once it is bent, and looks funny, add a morph to it at that point to clean it up. Then only apply the morph when it is bent. You can even animate this way, just keeping in mind to add the morph and take it away when you bend and release the bend.

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