I did not feel any problem at all, but my sister had strong headaches After we left Cusco.
Machu Picchu by Helicopter
A helicopter service from Cusco to Aguas Calientes is now available for those who like to pay extra money to get maximum comfort. This service was provided by a 12 passenger helicopter by Heli- Cusco.
Don't be confused, it may be a ingredient of cocaine, but still Cocoa leaves is perfectly legal to buy in Peru. But it is illegal in most of the neighboring countries, so don't carry the cocoa bag with you while leaving Peru and entering another country.
I hope you know that altitude sickness usually occurs when you are above 8000 ft. It is certainly not a hard and fast rule though. The effect of height depends on individuals. Many people feel nothing even when they are at that height. But there are cases when other people become sick before they reach 8000 ft. One important issue is the height of the city you live in. If you live in Denver, Colorado of USA, then you should not be much worried of Machu Picchu (elevation 8000 ft), since Denver is at a altitude 5900 ft. I have listed a table below with heights of USA cities.
I have also listed the altitude of the major cities in Peru. If you are visiting Puno or Lake Titicaca earlier than Machu Picchu, then you don't have to be worried about altitude sickness, since you should be well acclimatize in Puno. Also you have to reach Cusco earlier to reach Machu Picchu. Since Cusco is with higher altitude, if you stay there at least one day, then you should be ok in Machu Picchu.