hey JrWho

Try inserting 2 edgeloops round the corner edges (like if you performed a bevel but doing it manually helps to stop some of the little bugs that in my useage of bevels can happen), you can then use the slide edge tool to move the edges and control the fall off of the edge.

I would also create a smooth proxy as well as switching between the smooth preview, as sometimes when you using the smooth preview it can be a bit misleading as to where the geo that your moving actually is. Where as using the standard low poly and a proxy you can see if you need to add geo to areas when the edges get too close/overlap which is hard to see in the smooth preview. Also keep the low poly nice and clean.