Nothin' better on a hot summer afternoon than a slice of cold pie... And nothin' says "summer" like a great big fluffy meringue topping over that slab of rhubarb/lemon/banana custard.   Unfortunately, mine haven't been turning out that way. They bake up nicely, but fall within a half hour of baking.   I whip egg whites mixed with 1/8 tsp cream of tartar (per egg) until they reach soft peaks, then mix in 1 tablespoon of sugar (per egg) while continuing to beat until it gets stiff - then spread over the hot pie filling and bake for 10-15 minutes at 400F. Topping doubles in size, then falls as it cools, ending somewhat less impressive than it started.  Any suggestions? 
Piet Huysentruyt (a famous Belgian chef) advices to break and split your eggs, keep the egg whites in the fridge for a day or two, get them out and use them if they reach room temp.  And yes, in meringue, moisture is your enemy! It can help you, if you bake the meringue, to put a wooden spoon between your oven door, so that the vaporized moist can escape through the narrow opening. 