Aside from the Ki building aspect and the moving of center, which have been excellently covered by ravens and Ledyard sensei, there is an interseting exercise which I am sure many are familiar with. Begin with the arms extended, have you partner grasp your wrists and them move your hips backwards until your arms and your partners arms are properly connected. At this point continue back, sinking slightly, and your partners whole weight should be moved with virtually no effort. Then reverse the process, let your partner hold your wrists at your hips and begin to move your center forwards, exhaling as you go. As you reach almost full extension and your knee is almost above your toes allow your arms to continue forwards, maintaining the energy of your center's momentum. At this point your partner will be really moving. This is very similar to how some Chi Kung exercises move into applications within Chen style Tai Chi. As is the case with all Aikido the true difficulty lies in transferring this practise into your henka waza.