If I could ice every cake in whipped cream, I would. But, because it is whipped with air alone, it doesn’t stay thick over many hours. One way to keep it stable is to dissolve 1 teaspoon of gelatin in 2 tablespoons of boiling water, cool it to room temperature and drizzle it into your whipping cream when it is halfway thickened. Then, whip it a little longer than usual — until it holds medium-firm peaks.
Updated in 2019: I’ve been remiss to add the simpler stabilization method I’ve used for the last several years. It’s adapted from Nancy Silverton and it never fails. For every cup of heavy cream you whip, beat in 1 to 2 tablespoons of sour cream or creme fraiche. It won’t make it tangy (or not in any truly notable way) but it does keep the whipped cream stable. I have had jars of this in the fridge for 4 to 5 days and found them just as fluffy as they were on the first day.