The trick here is to find a balance between going slow enough that you're making a smooth edge, but not so slow that you make mistakes and burn out portions you don't mean to. It also helps to do the more detailed work first, as the gun heats up and you lose a bit of control. (I'm wishing I had done the central motif first). It also helps to have a secondary light source so that you're never working in shadow--it's very easy to lose track of where you are in the pattern as the marked edges burn away.
I had some black paint from last year (need to get more) and put some black in the crevices. It's important to get full coverage on these--any white (or in this case pink) styrofoam can be glaring if not covered up properly, so I try to make sure to tilt it periodically from different angles so I can see any missed sections or cracks.
Another plaque done: Justin Pieces *snicker*