Photos and video walk-around at pick-up and return are essential. I did a recent return at INV and helpfully the Sixt staff member saw me do the video, which I think intimidated him a little.

In these situations the car hire company always bears the onus of proof to prove the existence of the damage and that you caused it, so if you don't have some form of your own insurance to fall back on I would advise denying the claim and demanding evidence to the contrary. If you want to go to more extreme measures you could also consider cancelling the CC you used for the booking so they have nothing to try and charge any fees to.