On a related note: My car locks all of the doors when I lock the driver's side door using the key. I make a habit of doing this every time I exit the car, since I have owned it (~3 years).

I have only been locked out once; the one time I was locked out was because I didn't lock the car on the driver's side! (I left through the passenger side to avoid 5 seconds of rain. Bad tradeoff.)

Another car thing I do mostly for habitual reasons is turn on my lights anytime I'm in the car, even during the day; this makes me less likely to forget at night, and has the bonus of being very helpful in certain daylight lighting conditions.