Take some Diazepam or other sedative. Put lots of nice relaxing songs on your phone (that are available offline). Watch loads of Aircrash Investigation shows....this helps as it shows how much of a freak occurrence planes coming down are, and that they never due to turbulence.
Then just chill. Comfort is key. Turbulence does not bother me as I know what it is.
My worst nightmare is actually having screaming kids or a bunch of drunks on a plane, this would make me unsettled and nervous. Or worst still someone knocking the back of my seat, this would almost certainly result in an air-rage incident/decapitation.
All that said, you still wouldn't get me on one of those shitty little planes with propellers.