Disclaimer: not sure if your question was about using orbits to defend specifically or just defense in general.
Decentralization is probably your best bet.
The best way to not get hit is to not be a target. Becoming spread out and therefore costly to send RKV after RKV to take out each node in your societal network will become insanely costly and assuming each node in a societal network has the capability to launch RKV back, this also sets up the premise of mutually assured destruction via second strike capability.
If you have the ability, having societal nodes not just orbiting planets in a solar system but regularly entering and exiting orbits and slingshot-ing around the solar system (in planetary and/or asteroid orbitals) or even galactic systems would allow for much more chaotic movement if you must defend by orbital maneuver. (though in a solar system, if they just RKV the sun, then the whole solar system is done for. The saving grace is that your adversary would need an RKV at a magnitude to actually consider being able to kill a sun)
For sun defense, changing asteroid belt to asteroid shell or belts and use of something similar to the oort clouds as a dust shield I guess could be a good option.
(Main point done, tangential thoughts ahead)
I'd imagine there would be normal communities on the grounds of planets and in simple orbit around celestial bodies to be able to help resupply each node but ultimately if each node has a self perpetuating ecosystem with only the sun as an input that'd be useful. And probably blackbody thrusters.(iirc bb trusters are aka Nuclear-photonic thrusters?)
Whichever way this society is going to be governed is going to be an issue. Each societal node must be highly self sufficient due to all the moving about. I was imagining some sort of Neo-Feudalism or something more akin to the EU or UN.