Try to push your heels down with every stride, not enough to where you apply so much pressure you pop out of the saddle. Also sit on your butt bones and relax with the horse. Let your body move with the horse, if you are too stiff you are also more inclined to pop out too. And never give up once you have this down other things will come easier too :)
Some saddles are hard to sit because they're not really made for that purpose, usually deep dressage saddles are great. It also depends on your horse and it's stride, it took me a few weeks to get used to sitting the trot because he's very bouncy and only now after riding him for almost 4 years do I trully sit comfortable in the trot.
Trying to sit the trot without stirrups helps and just feeling the horses movements. Basically the posture of a lady with the hips of a whore!
1) if your saddle is to small for you than it might be hard to sit deep
2) you don't want to hold completely still you want to move with the horse a little. If you get stuff then you will bounce around but if you stay relaxed and move with the horse then it will help.