Clean the threads to remove all traces of oil. Put the bike in gear and lock the rear brake. Put Loctite on the threads and tighten the nut as much as you can.
Most importantly, you need the correct tool to turn the cush drive nut. Bashing it with a hammer and drift as many have done, will not work.
Several years ago I had an EBay supplier make up (FOC) a laser-cut tool to allow a 1/2" drive wrench ie. torque wrench, to be used. The quid pro quo was that he was open to sell the product on EBay. It works well but a quick search of EBay now doesn't show up the supplier at all. I'm sure that any other of the various sheet metal 'laser-cutting' suppliers on EBay could do the same thing. If you're local-ish to (Hampshire) you're welcome to borrow the tool.
