From what I read before doing long haul flights, sleeping tablets are NOT recommended as it is easier to deal with the effects of jet-lag if you have not taken them. Don't ask me why but I would imagine it would have something to do with drug induced sleep rather than natural kipping.
Other than that, I recommend staying clear of alcohol as it has a dehydrating effect which when combined with the already stale, dry and recycled air on planes allows for a particularly bad hangover/ poorly state on arrival.