We've found that the best way for us is to use a strip of non slip matting (the type you put under rugs or line cupboards with to stop everything sliding around en route).
We place the strip on top of the clothes rail and place the hangers on top of it. It's cheap and works a treat.
read somewhere that pipe lagging fitted over the rail,being larger and gripping keeps the hangers on.not tried it but the cost is worth a go.been looking for the hotel ones myself that have a hole at the top that the rail slides through.there was some on ebay a while back.
------------- the only silly question is the one you do not ask.
Just bend the wire hanger hook with a pair of pliers so it forms a circle around the rail...or pack your clothes in the boot of the car until you get there since heavy clothes swinging about in a wardrobe could cause instability
I'm with Purplehat on this one. My OH and two daughters always manage to fill the hanging space until nothing can move! So we have never had a problem with things falling off.
Get a bungay tie - the stretch elasticated rope type stuff with a hook on either end. Simply wind it around the neck of the hangers and clip the hooks together!
Screw an eye about 6mm above the rail at each side of the wardrobe. Cut a piece of curtain wire and fit a hook into each end. The wire should be cut to a length tah needs only about 10 mm stretch, otherwise it may pull the eyes out of the wardrobe wall. After loading clothes onto the rail, fit the curtain wire in place, above the hooks of the hangers. Then just remove and store it on arrival.