My custard for a banana cream pie didn't set enough. It was delicious but runny, and I'm wondering how I can make sure it firms up next time.

Sometimes if you use cornstarch in your pastry cream and it's cooked too long it won't set, at least thats my experience. When making pastry cream for a cream pie I use 1/4 cup cornstarch or flour to 2 cups milk or cream and 4 egg yolks. I have had pastry cream fall apart when I went past the point of no return when cooking it and it was soupy. In my experience it usually takes about 2 minutes to thicken sufficiently.