Also, by the same token, storing the meringue with any other food items that may contain a little bit of moisture will cause the meringue to soften.
Undissolved sugar will cause weeping as well, so a good way to prevent that is to be patient and let the meringue rest, and the sugar to absorb, then return to it and whip it some more.
Overbaking can cause moisture ‘beading,’ as the proteins tighten up in the oven and squeeze out moisture, which in turn then gets absorbed and makes the meringue soggy as well.
Make sure to spread the meringue thoroughly over the top of the filling to seal it up at the edges, to prevent shrinking, which can lead to weeping as well since the moisture in the filling is unlocked.
There’s definitely a trick, or several tricks, to making meringue work out right, and nobody gets it perfect the first time, but if you follow these basic principles you will undoubtedly be well on your way to magnificent meringues.
Have fun baking!