whipped cream icing
I agree with the butter fat and cold temperature issues previously discussed. You can also help stabilize your whipped cream frosting using 1 tsp. unflavored gelatin to each cup of VERY COLD whipping cream. A little powdered sugar (a pinch or two per cup of cream) will also help.
technically it would be frosting, not icing (which is typically thinner in consistency). anyway, it's all in the temperature. *everything* - the cream, the bowl, the beaters - has to be extremely cold.
most basic recipes call for heavy whipping cream (36-40% butterfat), sugar, and flavoring (e.g. vanilla or other extract, cocoa, fruit puree). whatever recipe you use, combine the ingredients in the bowl, cover it, and chill the bowl with the ingredients AND your whisk or the beaters from your mixer for at least 30 minutes in the fridge. to make the frosting, whip to stiff peak stage.