This method seems obvious but it only occurred to me a couple of years ago (which says more about me than anything I’m afraid…).
You can use this method for adult bikes but too but it works really well for getting kids’ bikes in the back of your car.
What you do:
simply loosen the but at the top of the stem tube (or it might be an allen key nut).
Then twist the loosened handlebars until they are 90′ the wrong way, making them in-line with the rest of the bike.
If you were feeling energetic you could take the pedals off too. Or better still, train the owner of the bike to do it all themselves!
Don’t forget to straighten them firmly at the other end (damn, that’s another 5 seconds…).