I packed mine over the last week. Make sure you have some comfy clothes and basic hygiene products, although the hospital can provide them if you don't bring your own.
Some other good items include chapstick, suckers, mints, slippers, iPod, massage oil, phone charger, camera.
Also, it isn't a bad idea to have a towel and a empty trash bag in your car to lay on the seat if your waters break before you get to hospital
I took initiative to get ready after a girl I know had her baby 5 weeks early. I thought, wow if that happens to me, I am NOT ready.