There are three specific stretches for this, that I teach. You don't just want to be able to "get" inverted. You want to be able to "fight" inverted. There is a tremendous difference. I start all of my berimbolo seminars and privates with the same three.
The first is the half-plow. It's like a plow, but your knees will stay on one side of your arms while you try to touch you far shoulder to the mat. Do both sides. You will hold these positions for 10 seconds on, 10 seconds off. 5 rounds.
The second is the reverse press. It starts off as a standard plow, then you progress by extending your toes to the ceiling, until your entire body is vertical and you are balancing on nothing but your neck and shoulders. Hands by your side. Slow rise to a 10 second hold with slow decent to 10 second rest. 5 rounds.
The third is a stretch my guys call the inchworm. You will begin face down with your body straight and hands by your sides. Turn the head to one side. Then, while keeping you knees straight, bring your hips up and rotate your head under them. You should now be completely inverted with your toes on the floor. Then, follow that motion through the other direction until you are back in your resting postion. Turn your head the opposite way, and repeat. 5 on each side with a 10 second rest between movements.
These are the drills I give my guys that want to increase their mobility and gain the ability to fight inverted.