You want your tweeters at the same level as your ears. The L monitor, the R monitor and the back of your head should form an equilateral triangle. Try to position the monitors so that the woofer is not exactly midway between the ceiling and floor, and you ears are not exactly between the front and back walls, or side walls.
EDIT: look in your monitor manual, but most speakers are designed to sound their best upright. If you do lay them down, put the tweeters on the outside and the woofers in the middle.