Laying down, make sure you measure your hands out on the bar so that your hands are even. There are usually marks on the bar that I line my pinkies up with. You want to press the bar up above your chest. Many people end up bringing the bar above their head (recruiting the stronger shoulders). Lower the weight if this happens. Bring the bar all the way till about an inch above the chest (don't let it touch), and then press back up.