Most of it is just being able to control every part of your horse's body at all times.
You should be able to move your horse's shoulder, hindquarters, and ribcage all independently of each other, using your legs and body. (If you want more details on that, I can explain further.)
And you horse should be soft in the bridle and easily give his nose in both directions, as well as break at the poll.
Circles are a great way to get a horse engaging their hind end, but it is important that you do "perfect" circles.
Dena Kirkpatrick has a lot of free videos on Youtube. Here is one of them that can get you to the rest of them. This video, she is using her previous "perfect circle" training to help the horse around the barrel, using his hind end.
In the video, you can see that the horse's outside hind leg, somewhat crosses over his inside hind leg. That's means he's using his butt.
The other exercises you mentioned are great as well, but again, make sure you are executing them "perfectly". Don't allow your horse to swing their butt on any exercise -- always make sure they are engaging the hind end.
You can also set up more barrels to do varrying patterns through them. Or if you've only got 3 barrels, you can turn them all to the right, all to the left, etc. You can work on fundamentals of the pattern, without actually doing the pattern, to keep things fresh for your horse so he doesn't get bored.