A pretty basic guide to AMS (acute mountain sickness) from Backpacker Magazine.
In my experience at least, eating regularly (i.e. stuffing granola bars into your mouth whenever you stop for a 10-min break), drinking lots of water, and not pushing yourself too hard helps keep altitude sickness at bay. I think most people experience some mild symptoms/discomfort when they’re above 14,000 ft though.
I’ve only had one somewhat bad experience with AMS (severe headache/ disorientation/nausea) and that was during my second day on the Inca trail, hiking the Salkantay Pass. We’d camped above 13,000 ft the night before and had to reach the highest point of the trail (15,000+ ft) by noon so we spent most of the day above 14,000 ft. I think we hiked around twelve hours that day (we spent the last couple of hours hiking in the dark). I pushed myself too much in the morning and what started out as a small headache just got worse and worse as the day went on. I usually try to stick it out and not take any medication, but when our guide told us we still had two or three hours to go before reaching camp, I decided it was time to take some Tylenol. At the time, my head felt like it was going to explode so that wasn’t really fun. No vomiting though! (Yay…)
One thing that I’ve come to realise though is that AMS not only depends on the person, but it also depends on the day. How fit you are or how experienced you are with fourteeners doesn’t necessarily matter. Sometimes it hits you and sometimes it doesn’t. A few days after Salkantay, I hiked El Misti near Arequipa and reached 17,000+ ft without any real side effects. So I guess part of it is just luck of the draw… but hopefully the link still helps!