High Altitude Landing Pushbutton, among other things
There is a high altitude pushbutton on the ovhd panel. This makes the oxygen masks drop at 16000' instead of 14000, plus cancels the excess cabin altitude warning at 9550. I think you get it at a higher altitude, but I am not 100% sure, I stopped flying the 320 a few years ago. I think you also get the Cabin Altitude Ecam Advisory a little later. The use of this PB was under the supplementary techniques, and it was required for operations to airports at or above 9200'
Regarding training, we had a special airports operations ground school when joining the company, then you would finish your normal line training, which includes many sectors to this kind of airports. After that, we were all qualified to operate in and out of these airports.
About the special preparation, that was just up to the pilot. Normal regulations regarding alcohol and rest applied. Sometimes you could feel dizzy and tired, specially after the walk around, and then you could use your oxygen mask to recover during the short turn around. You could take off and land with the mask on, but I have never seen anybody doing it, and it was not a requirement, at least not when I was flying there.
To finish, I remember it was considered a special operation, but we used to go there many times per week, so I could say it was a special operation that was practiced very often (at least 6 times per week).
My comment is based on the experience I had from 2004 to 2006, so it might be a little different now. Anyway, I hope I cleared some of your doubts.