Chain a wheel / wheels to a ground anchor - Make it visible to all
chain seat to steering wheel , covering it - again chain showing
Get a big dog
anyone wanting a mini enough will Hiab it away so box it in tight or hide it away
look for trackers fitted on it by others
there used to be a device called a wasp that bolted to the steering wheel
It screams when moved - can't find one now
If no-one see's where it is parked , then it is less likely to be nicked
Probably chain the front subframe to a ground anchor, a wheel could be removed, but even then they will bring a battery disc cutter and in 15 secs the chain is gone (see bikes stolen on youTube) and they will be delayed only 15 seconds however they planned to steal it.
Tracker a good idea but I've seen programmes where they aren't foolproof - a mini is small and not many places to easily hide one (weld it in the sill but a bugger to change the battery)
It depends how long you've got each time you want to use the car, as a scatter gun approach would add the maximum delay time (may not stop it altogether) kill switch, fuel cut-off tap, remove dizzy cap/switch leads/remove rotor arm, wheel clamp, steering wheel lock, audible alarm, screecher in the car, immobiliser, tracker, remove one wheel and lock it in the boot, chain it to the ground, 240v wired to the door handles (okay maybe not the last one!) and none of these prevent the car being lifted.
Locked in a garage has got to help but garages can be out of sight so not always I guess.
Car with a choke is going to confuse many younger tea-leafs....
Get good insurance with agreed value and don't get too emotionally attached is my best advice.
All these things will deter the casual thief/joy-rider but ......if someone wants it enough they will take it (or try quite hard)