I struggle a lot with this, as well.
Some conditioning tips I have are to work on your fan kick, as the movement is similar (and a nice, high fan kick can look frigging awesome).
There are also holds, in which you simply hold the pole as if to invert, your hips in front, pick up your legs, and see if you can bring your shins parallel with the floor. It's quite difficult, but it works a lot of similar muscles as well.
A lot of core conditioning, of almost any kind, would be super useful, as well as anything that works your shoulders, arms, and especially your lats (the muscles you use to pull your shoulders down, away from your ears).
Otherwise... frankly anything you do for strength and practice will help. I mean, one-handed spins will help, since it works your shoulders. Fan kicks, as I said. Floorwork, because that's a lot of core!
And don't be afraid to practice your inverts as conditioning for better inverts. The best way to get better at a skill is to practice that skill often!
Best of luck :)