I really can't. I tried but it doesn't work out well. I can close my eyes and kind of do that cat-nap thing, I am awake but settled down like I was sleeping. I have flown to Rome from DFW once (8.5 hours I think), and I tried to sleep, but I just couldn't do it. The entire airline seat things may have had something to do with it.
BTW, if anybody plans on flying at least eight hours, I think it is well worth it to upgrade your seats so you have more room.