The trick that I developed is to twist the valve on the pad closed, then hold the pump sack (I'm using the bigger one, not the included one) up high, then fluff it up to fill it with air. Yank the draw cord closed, then twist the valve on the pad a bit open, and then roll the pump sack down. When it's fully rolled up, twist the valve closed again.
I've found that for the first few pumps, I don't need to close the valve between pumps, but in order to get it to be relatively firm, you need to prevent the air from escaping while you're refilling the pump sack. It took a bit of fiddling at first, but now it's pretty easy. For me the finicky part is closing the valve on the pad enough to avoid losing air while refilling the pump sack.