Get a magnetic key case. Hide it well.
Carry a spare in your wallet or purse.
Get in the habit--any habit, whether it be an automatic pat down for your key whenever you exit your vehicle to never locking your doors from the inside of the car by always using the key to lock/unlock.
Keep your keys on something incredibly convenient to use such as a lanyard or carabiner, and again with the 'habit of use', by connecting them to yourself or something you always carry.
Get a combination lock for your car door. You know if you are the person who needs this.
But, just for the sake of argument, say your magnetic key case falls off, you lock your wallet in the car, you break your habit, your lanyard breaks and you forgot your car combination--what are you going to do?
Or, in the words of a famous movie, "Who you gonna call?"
As a last resort option, take the next fifteen seconds of your life and add the name of a quality locksmith who specializes in 24-hour emergency lockout service in your area to your phone contacts.
My own locksmith service, Kwik Pick Lock and Safe (623-300-1889) not only specializes in emergency lockouts, but will come to a lockout FREE if there is a child or pet locked in the vehicle. God bless you, Bill.