Always is correct - you need to relax and let your legs hang down to sit.
1. See if you can find a dressage saddle that fits you and the horse. If it is a good saddle then it will help guide your legs into the proper position (knees/thighs pointed NOT straight ahead but towards the ground out on front of you).
2. While you're looking for a saddle like this you need to work on getting the horse to step underneath itself with it's hind legs and reaching forward into the bridle. Use (long) legs to encourage horse forward then half halts (HH's) to keep horse from running forward so that instead horse takes bigger steps (more over-track).
3. Keep rein length so that horse has head slightly above the vertical yet nose is only slightly in front of the vertical AND there is an "arch" in the neck. You know the arch is correct when you see the middle of the neck on both sides have a "bulge" to it.
4. Lastly once you have 1-3 done then start sitting a few strides at a time until you bounce too hard, then go back to rising until you're ready to try again. If 1-3 are correct (done correctly, saddle fit is correct and a good saddle) then you'll be able to sit. Gripping with legs (thighs or knees) will cause you to bounce harder. Also - lifting ribcage when you would "rise" in the post helps you to sit easier, as does looking straight ahead - NOT down.