I agree with
@WelshGas that it starts with key management. I do think (or, more correctly, guess) that further disincentives may add something too: the more unusual/unique the better. Previous posting on the subject have talked of recognised brand wheel clamps and steering locks being removed in seconds by ‘professional’ thieves.
I use a high quality 1.7m long German motorcycle heavy chain/lock around the steering column and brake pedal with harmonised custom brand printed security bands on the door handles (when parking in dodgy areas) creating I hope a puzzle of apparent additional security that leaves a potential thief wary - though I am realistic that it is a gesture and if an expert thief wants my California they will not be deterred.
I have mused about a swivelled seat lock, and subject to the caveats above it may have merit, the more obvious (and unique) the locking the better.
Always parking at steering full turn, back to a wall/obstacle, with the steering lock engaged, to make towing away or low loader theft more difficult, again as others have posted, probably has merit too.
My first car, a 1933 Austin 10 had a switch for ignition and did not even have locks on the door handles. My security in those days, was to open the bonnet and pocket the rotor arm.
I also carry in my California a battery pack angle grinder, in case I find I have been locked in ‘by a mistake’ behind a farm gate for example. So quick and powerful are these now I imagine they will defeat many physical protections.