I've got trays I brought with me from the UK and just go and buy the whitening solution from my dentist whenever I feel the need. Probably every six months or so.
I get six syringes and one syringe does two nights as I leave mine on overnight.
I have.. On several occasions :) I've done ZOOM whitening once which was very effective! However, my teeth are kind of sensitive so I had striking pain for 1-2 seconds a few times the following two days. I also had trays made that I can use at home when the teeth get yellowish (I use them every six months or so for an hour a day, 3-4 days in a row each time) :)