My advice would be:
1) get there as early as possible. It is no longer possible for hikers to wander through the Sun-Gate at 6am to get first sight. Instead you have to come through the main entrance which opens at 7am. Buses run up from Aguas Calientes from before the crack of dawn, so if you are not a hiker seriously consider travelling up the day previously and staying overnight in the town before visiting the site first thing. The train from Cusco arrives around 10am—suddenly the place is overrun with tourists around 500,000 visits per year!). The earlier you arrive, the quieter the place will be and the more chance you have of exploring in detail. Once you’ve seen everything from up close you can then wander as you will, or just sit up by the Watchman’s Hut for an hour.
2) Go to the toilet before you visit. The only loos are by the entrance gate, down a steep flight of stairs.
3) Take a tour guide to explain the significance of what you are seeing, otherwise it could all just be chunks of rock to you. We were lucky in that our guide from the Inca Trail, Julio was with us, armed with his trusty notebook. He knew the history and significance of the site off by heart, and was able to answer most any question, but the reproductions he had of Bingham’s original expedition, showing the mountain peak thick with jungle really impressed me. Take more camera film than you think you would need.
4) Take water – the only shop is by the monstrous Hotel Machu Picchu and is terribly overpriced. If staying the whole day you would probably still save money by getting the bus down to Aguas Calientes, eating there, then returning.
5) And finally, if travelling by train, bear in mind that it takes four hours to / from Cusco, but only two from Ollantaytambo, which is a mere hour from Cusco by road. You can save time by getting a bus or taxi to Ollantaytambo, then a train to Machu Picchu, and vice versa.