You need to cook the shells beforehand and make sure they've had time to cool. If in a hurry, run cold water over them after draining. Don't overcook. Keep it al dente.
We made it again this weekend for some friends. This time we made a few modifications:
-instead of 1/2 tsp. salt, we threw in an entire 1 tsp.
-since there is a lot of cheese filling, we stuffed 25 shells instead