Recent growth in adventure travel has made trekking at high altitude simpler and more accessible, with the result that more and more people who go trekking are ignorant of the basic facts of altitude illness. 

Incredibly, I have known people who have hired a horse or a yak to go up higher when they were too sick to walk. This is courting disaster. 

It is amazing how striking and dramatic the relief may be with even a couple of hundred feet of descent. People with signs of HAPE or HACE have to descend.

(perhaps like the buddy system in SCUBA diving). This rule gets broken with unfailing regularity every trekking season in the Himalayas, because people are just too anxious to complete their trek, even if one of their party members is ill. A trekker with AMS, HAPE or HACE will want nothing more than to be left alone, unbothered, at the same altitude - potentially a fatal option. There is no alternative but to bring the person down to a lower altitude accompanied by a friend who speaks the same language. 

Going too high, too quickly, is the single most important cause of susceptibility to AMS. Beyond about 9000ft, the sleeping altitude should be no higher than about 1500ft from the previous night's altitude. The sleeping altitude, not the altitude achieved during the daytime, is what is important. Altitude sickness often manifests at night because during sleep the oxygen level in the blood may dip further. Many mountain climbers will have been to 14000ft or higher in the Alps or in North America but few will have slept at that altitude. In the Himalayas, you don't have to be an experienced climber, or use crampons, to be able to "hang out" at 15000ft or higher for days: easy accessibility to these altitudes makes exposure to AMS also much easier. 

While ascending, every second or third day should be a rest day for acclimatization. "Climb high and sleep low" is the dictum, but it is important not to exert oneself excessively in trying to fulfill this.

The trekker should not be in a hurry in the mountains. The itinerary should be planned so that there are enough "leeway days" in case more time is needed to acclimatize. Trying to do a high-altitude two-week trek in one week is always fraught with problems.

Excessive physical exertion at high altitude makes one more susceptible to AMS. It is important to take it easy at high altitude, especially in the initial days. 

People who are very fit, for example marathon runners or those who carry very heavy backpacks seem more vulnerable to AMS than others, probably because they push themselves harder. I once looked after a trekker who felt he could not break his morning jogging sessions despite a strenuous trek day ahead, even at 4000m! The feeling of "man against nature" may be stronger in this fitter group. 

Jim, a rock star, decided to "whoop it up" with four bottles of beer, on arrival at 3500 meters in the Everest region. He felt ill with severe AMS and needed to be helicoptered out two days later. He had been warned not to drink alcohol on the trek, especially while ascending. Alcohol may dehydrate the trekker but more importantly it depresses breathing or ventilation.
Sleeping pills may have a similar effect.

Adequate amounts of fluid (about 3 liters a day) are necessary in the mountains - dehydration mimics altitude sickness and may even predispose to it. On the other hand excessive water drinking should also be avoided as this may lead to electrolyte imbalances. 

A high carbohydrate diet aids ventilation and efficient use of oxygen. The good news is that - in many high altitude places - there is not much alternative: rice, potatoes and other starch-laden foodstuffs tend to be the staple, with not much else to choose from.

Diamox (acetazolamide) may be necessary for people going on rescue missions at high altitude or flying in to high altitude cities like La Paz or Lhasa. People with sulpha allergy should not take diamox, the primary drug for prevention, and further details are given below. A second drug, dexamethasone (see below) should also be  carried, particularly if the destination is remote:
this can be life saving if HACE supervenes. 

Ginko Biloba doesn't work in the prevention of AMS