The best way to prepare physically for the climb is to make regular hikes, building up the frequency and length towards the date of travel (but ensuring that one is not overexerted just before leaving). Walking in hilly areas and on rough surfaces is great preparation, as is walking up and down stairs and step-aerobics.
All aerobic exercise helps to strengthen the cardiovascular system: swimming, running, cycling etc. are all good, as is any exercise, which increases / maintains the heart rate for at least 20 minutes. It is advisable to do your physical preparation in the boots that you will be wearing for your climb.
Go slowly! This is so important because walking slowly allows the body to acclimatise. The speed at which clients walk during the first two days of the climb will determine the chances of reaching the summit. The slower the better. A really good idea is to breathe entirely through the nose for the first two days of walking.
If it is necessary to breathe through the mouth during the initial stages of the climb, then the pace is too fast. Later on, breathing through the mouth will become essential, but (of course, every individual's metabolism varies).
Climbers must be prepared to recognise and respond to early symptoms of altitude sickness, which is caused by reduced levels of oxygen / air density. Any feelings of nausea, headache, fatigue or severe breathlessness should be reported immediately to the guide.
The human body is well capable of adjusting to altitude - the actual individual rate of acclimatisation is a genetic factor - but the process takes time and the best way to minimise the symptoms of Hypoxia is to ascend slowly - i.e if in doubt increase the number of days on the mountain!