My county junior coach was known for taping players hands to their rackets in order for them to play with the correct grip, unable to go back to old habits, until it felt natural and the tape could be removed. 

Another coach I had used to get us to pretend we had a rubber ring (the swimming floats that go around your hips) around us when we played, so our rackets were always in the 'ready' position. I heard 'rubber ring' shouted at me numerous times across the hall as a kid and I seem to remember somebody in my group actually being made to wear one because their racket carriage was so poor. 

Once my technique had been honed, I also practiced a lot of backhand clears, against a wall, with a squash racket. This gave me good power from a young age. I wouldn't advise doing this until the correct technique has been ingrained, so as not to cause undue stress to the body. 

A bit left field, but as a young child I also did a lot of gymnastics and was lucky enough to live right near to the national academy at Lilleshall. The balance, strength and movement skills I learned here I was able to transfer onto a badminton court and i believe this has played a major part in me never succumbing to injury in 12 years of competitive play at a decent standard. 

Rogerh – thanks, sounds like a good reflex exercise…sadly it doesn't involve mains voltages…..

Paul – I was discussing music with my coaching mentor; I would certainly like to use it to improve rhythm and footwork speed. I was going to use a sheet draped over the net to hide hand fed shuttles – again to improve reaction times and to prevent the player moving before the shuttle is fed. I like the bungee cord one – I was thinking about that the other day – how can I get doubles pairs to act as a single unit…I often talk to the players about being effectively joined by an invisible pole (a bit like the man and woman on the weather clock)