If you're using smooth poly or sub-d's you should make sure that all (or as many as possible) of your faces are quads, otherwise you will get some pinching in the areas that has 3/5 sided faces.
If your problem is that a curve straightens out whenever you add a new edge, it's because you need to tweak. The more vertices/edges you have in a straight line, the straighter the smoothed mesh will be, and when adding a line between two existing ones, they automatically end up aligned. Hope this makes sense, it's a bit tricky to explain
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