I've attempted this before.
My parents' house has a balcony - close to my brother's room, so every year I stayed with them in the past 3 years, I would sneak outside during the middle of the night and lay on the balcony. It was very uncomfortable, to say the least, but as time went on (and as it got colder) I would leave pillows and a yoga mat outside so I wouldn't be overly tempted to return to the warmth of my bed.
The only thing that's stopped me every time is that it takes too long, in the dry air. It's enough time for me to enter a moderately hypothermic state and decide I want to live another day. I've submerged myself in a lake before, and it wasn't too bad. It was uncomfortable as all hell, but it was the rain falling on my face that made me retreat.
A few things I always made sure of before I'd attempt:
1: Have a lot of time on your hands/not be expected anywhere. I always make sure that I'm not expected to be somewhere for at least half a day. This may seem excessive, but depending on whether or not you're gonna be in water - and the outdoor temperature - it can take a long time for hypothermia to even kick in.
2: Practice taking cold showers. This boosts my cold tolerance, so I try to do this at least a week before my attempt, to get adjusted to sitting in coldness. Be warned that from my experience, standing in a cold shower feels different from laying submerged in a pond in the December rain. If possible, go for the water; it effectively reduces the time needed to die by more than half. If you don't have 12 hours to spare, you can cut it down to about 4 to 6 in the right temperatures.
3: Make yourself as comfortable as possible. It's not easy being cold for at least half an hour (then you go unconscious, in the best circumstances). Have a nice meal, tie up loose ends, bring a comfort object with you; it's going to be a long, uncomfortable ride. But if you're looking for a painless method, this is arguably the most reliable one I've found.
4: Wear the right clothes. I prefer cotton, because they are not resistant to water and are the least practical for surviving hypothermia. Better yet, don't wear anything. One stage in severe hypothermia is paradoxically undressing. This is due to the fact that when the body gets too tired to constrict blood vessels (for keeping the core of your body as warm as possible), vasodilation will occur. This expansion of the blood vessels sends a wave of heat over you - like plunging your body in a warm shower - and gives the illusion that you're burning up. I found this sensation to be a relief, when it happened to me during an attempt on the balcony. I shedded my socks and thin layers of clothes until I felt numb.
5: Make sure the temperature is right. I never go for anything below 32 degrees F (0 C), because that's when frostbite can occur, which is painful. Anything above 50 F (10 C) is a no-go, due to time limitations.
One adage that gets passed around in wilderness survival guides and medical handbooks, regarding hypothermia, is this: nobody is dead until warm and dead.
Hope this helps someone. It took me a while to gather all this information from past attempts.