I try to mount the prop so that it will be horizontal when the piston starts coming up against compression. The idea is that, when the engine quits, the compression will stop the prop in the horizontal position and help prevent broken props in case of a bad dead stick landing. It may also protect the engine crankshaft not to mention making flip starting easier and safer.

Most 4-strokes have a double prop nut. It's just like a jam nut you find in lots of other applications. Usually the second nut is tapered and fits into a recess in the first or something similar to help them really lock good. The first nut can't loosen up because of the pressure created by the second nut. 4-strokes can backfire from time to time, especially if they aren't tuned well, and the extra security helps prevent the prop from flying off if this happens. My new OS .75-AX 2-stroke even has a tapered jam nut. It's probably the simplest, best all around way to lock a prop nut. You wouldn't want to use lock-tite since the nut has to be removed pretty often and things like safety wire or clips would be a pain too.

Another mistake I see a lot is not getting the prop nut tight enough. The crankshaft threads, the nut and the washer are all hardened so there is no reason not to put some real torque on it. After all, it's holding a spinning knife at 14,000 RPM.