To sign "ice-cream" hold an "S" hand in front of your mouth and move it downward twice as if you were licking an ice-cream cone.
ICE-CREAM:
See: Animation: ICE-CREAM
NOTES:
You don't actually stick out your tongue in a true-to-life licking movement. If you do, that would inflect the sign to mean something to the effect of "She took two big licks." If you are telling a story and that is what you want to say (She took two big licks..." then fine -- you can stick out your tongue).
But typically you will just see the sign done in one of three ways:
1) Slightly mouth the words ice-cream
2) Don't use any mouth movement
3) Use two very, very slight licking movements with the tongue
I've seen the sign for "ice-cream" done all three ways by native Deaf signers. I do it in my example above with no mouth movement. I just asked my wife (Deaf) how she signs it and she partially mouthed "ice cream." I showed her the version with two small licks and she didn't like that version at all, but she thought the no-mouth-movement version was "okay." If you see it a certain way in your area then certainly do it that way. Better yet, just send me a coupon for Baskin Robbins so I can do some more research on this topic.
Notes: