I twist on the spot. I have flash cards with choices, and when I switch over to balloons, I take a second to flip through them, and add/remove any, depending on time, number of kids etc.
I bring the birthday child a pre-made balloon birthday cake hat (and tell their guests that if I come to their birthday, I'll bring one for them, too!

), but everything else I make there. I'm not a mind reader - how am I to know who's going to want what, and in what colours?! Plus I hate when balloons oxidize.