No one knows what Silly String is really made of. All we know that is that it is hard to get out of fabrics and impossible to get out of vinyl. In fact, if Silly String gets on your favorite vinyl boots, you are probably stuck with the stain forever.
Remove it from your clothes the second you are squirted. This is the most effective way to ensure that the Silly String does not stick and does not stain.
Pick the Silly String off your clothing once it is dried if you were not so lucky to be able to remove it as soon as you were sprayed. The only instances that this will not work are if the fabric is vinyl or you were sprayed from less then 2 feet away.
Try using paint thinner if you were sprayed from less then 2 feet away. Chances are the fabric will have absorbed some of the Silly String component and is now stained.
Test the fabric in an inconspicuous area to make sure that the paint thinner will not ruin the fabric. You can skip this step if you feel that fabric will be ruined by the paint thinner.
Pour some paint thinner on a cloth. Make sure you are in a well-ventilated area that is free of any open flames.
Dab the cloth on the stain repeatedly until the stain is removed.
Let the shirt dry, then wash in the washing machine.
Dispose of the paint thinner as directed on the container.