Modeling geometric shapes with rounded corners

I’m a modeling noob and have been banging my head against this for a bit: what is the recommended approach for creating geometric shapes with rounded corners? (i.e. how to add rounded corners to any geometric shape while still keeping the overall flatness in its lines.) In my specific example, I’m trying cut out a trapezoid in a plane (think Nexus 4 speaker cutout) and when I apply subsurf everything goes all oval-y no matter what I’ve thought to try. Also, when modeling something like a smartphone where I want the overall result that subsurf provides, are there parameters other than crease that I should be thinking about for the exceptions. Pointers to any existing tutorials are greatly appreciated…

Try walking through some of the modeling tutorials at

In edit mode select the edges you want rounded, ctrl + B to bevel, settings on left are so you can increment number of bevels and profile. #. of bevels can be done with scroll wheel also.