Knock on is nice, but I have found something that works even better for me. It is the app "Gravity Screen on/off". It is available from the play store. It uses the level sensor in the phone to detect when you place the phone down, and then puts the phone directly into sleep mode. When you pick the phone up, it turns on by itself. I hardly ever even use the power button any more. Anyway, this works for me because I mostly use my phone at home and keep it face down on a table. Gravity Screen doesn't seem to work so well when you are carrying the phone around, although adjusting some of the settings might help with that.