simples!!! Just spend time with them, grooming them playing with their manes and tails and generally spending time with them.I have NOT included riding in that on purpose. Sit in the field and read a book and watch then. They gain your respect and trust from the work you do on the ground.  Lead them around the sandschool( if you have one) doing exercises with them doing circles, serpentines and shoulder in etc. they can all be done working on the ground. Take them out for nice walks and let them graze in safe places for a while.They learn from your voice and body posture what you want them to do and then when out riding they know from your voice and you body position what you want. 

Good luck and just pend a bit of time with them. xxx