I say 1, 2, 1,2 over and over again in my head.
Think of swinging your pelvis forward rather than up and down.
Having someone longe your horse and let you trot and post with your eyes closed helps you find the rhythm. Or if you're riding a horse that's going too fast (or too slow) I count 1 2 1 2 as I'm going up and down. Then you can count slower or faster and match your body to it.
saying up and down in your head or out loud while making the movements helps a lot.
combine both, in the first place you need to take up the horse's rhythm so let yourself be "pushed up" by the horse. That way you almost can't not have the right rhythm. If that's working then you can start counting in your head and try to influence your horse's rhythm activley through your seat and leg aids.
I would just say to never work to hard at it. Just flow with the horses beat, let the horse carry you and push you up out of the saddle. The harder you try to perfect it the more forced and stuff it will look. Good luck! 😊
My instructor told me to count 😂 works for me