I like to make signs, so when I saw an excellent instructable on making foam "sandblasted" signs, my ears perked up! I make them too, but have what I think is a simpler method. Check it out.
Step 1: Cut out your letters
In this step, you make the letters to be used for the resist. The letters prevent the foam from dissolving, much the same as in sandblasting. I use my word processing program to print out the message I want to use. The letters are easily cut out with scissors as seen in the picture.