For many, the Inca Trail is the most well-known of all pilgrimages: a walk along an ancient route that ends, on the fourth morning, with an opportunity to pass through the Sun Gate and watch the sun rise over the Machu Picchu ruins. I am glad I went, but wish I had known that at least 3 "Inca Trails" lead to the same destination-- and some are less well-worn, touristy paths than the one I took. For instance, there is the famous 4-day Inca Trail, a 1-day version, and the 7-day Salkantay trek.
Relatively recent government regulations impact the Inca Trail experience. The government caps the travelers to 200 people per day, with an additional 300 guides, cooks and porters. Each of the 177 tour operators in 2010 is licensed, and walkers must book through them. Therefore, there's a steady but limited stream of walkers in a 2:3 ratio with service providers-- an unusual and restraining situation for independent trekkers. On the plus side, as I found though Peru Treks, it's a rare treat to have someone carry your tent and luggage and fix surprisingly delicious hot meals.
The most practical advice is to make sure you give yourself time to get acclimated: I stayed 2-3 days in Cusco, fighting off altitude sickness the first day and then doing short half-day hikes on the second and third day, before starting on the Trail. Drink water, drink mate de coca tea, sleep... and, if needed, pick up an over the counter prescription.
Selected photos from my trip
Andean Travel Web overview of Inca Trails