I can't take credit for this one - learned this from my students (who maybe got it off Pinterest? don't know.) It's genius, though - the balloon is squishy enough that as the pot shrinks it compresses, so you can just leave it in, keeping the pot round as it dries.
To be fair I have not tried this myself - as I was typing the last paragraph, it occurred to me that the balloon *might* be squishy enough to distort right along with the pot, if say, the pot dried to fast or something like that.
Anyway, it's any idea. I will let you know how this student pot turns out.