When Making A Pie With Graham Cracker Crust, How Can I Get The Crust Just Moist Enough?
I've found by asking that most people don't bake the graham cracker crust before filling it. This is especially true if we're talking about a pre-made grocery store graham cracker crust.

It needs to be baked and "sealed" in the process and it will stay much firmer and hold up against the moisture longer.

I brush graham cracker crusts with an egg wash and bake for a few minutes until it toasts up a darker brown. The egg wash, when baked on, helps form a decent barrier against the moisture of the pie contents. This has the added benefit of a much better tasting graham cracker crust.

If you aren't used to doing this baking step, you'll probably be surprised how much better the pie turns out. You'll also start noticing how many unbaked graham cracker crusts show up at gatherings as it's immediately obvious the difference once you've seen both.