After several small floods, two medium floods, and one near disaster (finished water change and walked out the door to go hang out with a friend, halfway to her house realized "Oh crap, I didn't turn the water off."), I've learned to set the timer on my phone whenever I do a water change. No matter what I'm doing, I grab my phone and set the timer, religiously. (Usually 20 min for my 180g or 15 for my 135g). It's too easy to get involved in a good book or video game or whatever and end up with a baaaaad problem.