Like some people mention above concentration, visualization,breathing which leads to relaxation and meditation(mu shin). But most importantly to develop ki power is the use of your mind. Don't strive for it instead let it come and let your unconsicousness take over. It's really not so complicated.People and the media exaggerate it too much like you have to do secret moves and techniques and there are so many complicated stuff to reach to this level where you can control ki. Seriously... it's not complicated just keep things simple and you'll get the idea eventually. All it takes are research, discussing it with experts, correct techniques and time.