Wait for him to bounce you up in the saddle and then just rock your pelvis up and forward slightly. Not very far - and as soon as you get up, rock your pelvis gently back down towards the seat of the saddle. Think (or say) "up, down, up, down" in rhythm with his trot, and you should BE there on the word, that's when you arrive, not when you start.
Don't consciously stand in your stirrups. You don't need to get up real high in the air, certainly not to a straight leg -- only just enough for your bottom to clear the seat of the saddle. That's all it takes, you can always tell somebody who's just learning because they post WAY too high. That also makes it difficult to get back down on time - which makes you bounce.
Do not look at the shoulder for now, unless you hold your head up straight and just cast your eyes down to catch a glimpse of the point of the shoulder moving forward. If you bend your head over it will collapse your shoulders and ribcage and make it difficult to post. Posting on the wrong diagonal won't hurt Okie, it's better not to worry about that for now until you get the hang of the motion.
It will come to you suddenly, and it's like riding a bicycle, once you've got it, you've got it.
Just keep trying. A slower rhythm is easier than a rapid trot, so long as your horse keeps his impulsion and doesn't fall into a shuffle (nobody can post a shuffle!)
I told her faster was easier! LOL!
It is with Bay. His stride is really long I feel like there are too many seconds of air in between to post gracefully unless he really moves out. I have to pause too much. In fact, unless I open him up into his endurance extended trot on trail, I sit it.