Believe it or not, the BEST thing at cutting smooth lines on that stuff is an electric knife. Works GREAT.
If you dont have one, I like to use a knife that has a sharpe cutting edge like on some steak knives where the edge is not smooth, but very sharp. You can use a sawing motion and it works very well.
I flip the foam over and draw on the BACK side, then cut. That way the ugly marker line doesn't show when in the case.