If the best spot we find is still slightly inclined, first I make sure my husband is positioned below me because he is an active, and sound, sleeper and won't awake to fix his position if he slides somewhere. Next, and also when I am alone, I take something from my kit--shoes, the big first aid kit if I am leading a group trip, or even the little personals ditty bag, or a food bag (assuming it doesn't have to be sequestered elsewhere)--and tuck it slightly under my torso on the low side. This is not uncomfortable and provides just enough leverage to keep me from sliding. Once I tried a partially full Evernew (soft) water bottle for this, but it seemed to make a chilly spot. I am a back and side sleeper, and use a short ridgerest pad, and usually a tarp so I have lots of room to work with.
As far as finding the flattest spot, remember it's hard to tell just by looking. Before setting up your shelter, lie down on the ground to test it for subtle slope, and orient yourself the most comfortable direction for you. Most people seem to prefer their head at the upper end but some people (including the nurses at the health center where I went to college) recommend elevating your feet slightly if you have been on them all day. Also pay attention to what's on the ground. Pine needles are slipperier than broad leaves or grass.