I do it all the time too.  As others have said, I use some pieces of 3/4
inch plywood about 18" square for putting under the jack.  I love to use
cardboard, and even use while I'm working in the garage.  It is soft,
clean, warm, and easy to slide on.  I put a big piece under the van and can
just scoot/slide under there.  It doesn't raise you up like a creeper, and
if you are pulling on a wrench, it lets you stay in place and have good
leverage.  When you cardboard gets grungy, just get more.  My favorite
source is a local furniture store.
Edward