How can I get Italian meringue to thicken?
Try this: separate the white from the yolk 3 or 4 days before you want to make meringue. Keep the white in an hermetic box in the fridge and take them out of the fridge one hour before you begin. So the egg white foam will be more stable.

Another trick is to add some drops of lemon juice to the egg white right before you beat them.