How do you do posting trot without stirrups?

You have to have a good inner thigh to be able to post without stirrups. I can because I have worked really hard at it. You can practice without stirrups on a bouncy horse (SAFELY!) sitting, a slow trot/jog. then get back on your horse (which i assume is not particulary bouncy) and posting should be easier. remember not to accidentally kick your horse while trying to do so!