Yeah, try to take the "improvement in behavior" out of the equation. If you would normally say thanks, say thanks. If not, don't. Thanks for going out of your way to do something especially nice that you didn't have to do. No thanks for not puking on the side of the toilet or for not starting a fight at bedtime. Taking out the trash--meh. If you would tell anyone who lives there and contributes to the trash thank you for taking it out, then do. Otherwise, not.