It is crucial to spread the meringue over the filling while the filling is HOT. This will heat and seal the underside of the meringue so it cooks as thoroughly as the top (which is exposed to the heat of the oven). Don't overbake the meringue. Overbaking will cause water droplets to form as it shrinks. Undissolved sugar in the egg whites can also cause weeping. You can also try adding a mixture of thickened cornstarch and water to the egg whites to bind and stabilize the liquid in the meringue, which will keep it from seeping out.