One of the greatest world heritage sites, Machu Picchu is one of the true manmade wonders. The location is stunning, and the ruins are in excellent condition. However, like the review below there is cause for concern. Part of the problem is Peru is full of incredible Inca and pre-Inca cultural sites. Yet the majority of these sites are relatively unknown, this contributes to Machu Picchu being viewed as the only must-see site in Peru.
Peru has had little success within the world heritage program recently only gaining 3 inscriptions in the last 20 years. For a country that is extremely rich in cultural and natural heritage this is suprising. Raising the price for visitors to Machu Picchu seemingly has not managed tourist numbers, nor has the remote nature of the site. Beyond establishing reservations to the site itself, limiting the number of visitors (which may still be required), the only solution to me seems to be promoting exciting alternatives. Promotion of Peru's various pre-Inca cultures is something you just don't see. Qhapac Nan could be a great alternative to the traditional visitor experience in Peru. I look forward to its inclusion on the world heritage list.
Nevertheless, Machu Picchu is probably not in as much danger as it may seem. First, the site is indeed huge, even with a large amount of tourists I never felt crowded. Secondly, the site is well maintained, and the buildings are entirely built from quarried stone, or built on natural rock. This makes the site largely immune to the large tourist flow.