According to Cooks Illustrated, meringue must be spread on top of piping hot filling (and never on a humid or rainy day) and they suggest adding a small amount of cornstarch. There meringue formula is as follows:
1 Tbsp cornstarch
1/3 c water
1/4 tsp cream of tartar
1/2 c sugar
4 large egg whites
1/2 tsp vanilla.
Whisk cornstarch and water in small saucepan, bring up to a simmer and begin whisking until thickened. When translucent remove from the heat. Let cool while beating the whites.
Combine sugar and cream of tartar and set aside. Beat whites and vanilla until frothy. Beat in sugar mix, 1 Tbsp at a time until the mixture forms soft peaks. Add the cornstarch mixture, 1 Tbsp at a time and beat until you have stiff peaks. Spread on hot filling and distribute evenly sealing at the edges of the crust. Bake until golden, about 20 minutes in a 325 degree oven.
I find it's worth the few minutes extra time to do it this way.