Good tips here, tapping yourself is probably a good way to do it, but how I made it a habit was just working consciously on always returning your hands to the proper position every time you throw a punch.
I've done a lot of bag-work while doing this, and it doesn't take that long until it sticks without having to think about it in my experience.
Fact is, I did boxing since I was 14, did that for 8 years or so before I started doing karate. Now it's been all karate for the last years, and I STILL can't shake the instinct of returning my hands to proper guard, instead of the lower/different guard I'm supposed to use in karate. So yea, it's all habit, and with some proper coaches/partners, you'll quickly be reminded of when you forget.
