A lot of people come well prepared for the hike on the Inca Trail, but I have seen many examples of problems due to inadequate preparation getting in the way of complete enjoyment of the Inca Trail. The most important thing to do is acclimatize to the high altitude. Coca leaves may help, but there is no substitute for spending a few days around Cusco to get adjusted prior to beginning the hike. There are several short hikes around Cusco (see my Cusco journal) that serve well to prepare for the Inca Trail.
Equally important is the issue of what to bring with you. Here are some suggestions that I think are particularly applicable to hiking the Inca Trail. The golden rule is to travel light, so plan carefully. Bring clothes that can be packed easily and that are warm but light, such as fleece. Do not wear anything that is military clothing here or elsewhere in Peru, as people may mistake you for a soldier or a terrorist.
An absolute must is a pair of good, broken-in hiking boots. If they are not waterproof, be sure to waterproof them with a waterproofing agent. Also critical is a waterproof jacket. With the exception of a couple of months a year, rain is very common along the trail and wet clothes do not easily dry. Waterproof pants are also recommended. Bring a waterproof cover for your backpack. It may prove to be the most important part of your gear.
Alternatively, pack your belongings in plastic bags. Make sure that you take a backpack that fits you well. Also, consider bringing trekking poles, even if you would never consider using them ordinarily (as is the case with me). They are particularly useful when descending along the uneven Inca steps after the second pass. A 20-degree rated sleeping bag should be sufficient but nights can be very cool. A hat is also recommended to shield from the bright sun at altitude.
Another recommended item is a good head lamp, useful for hiking after dark when you may need both arms/hands to maintain balance on uneven terrain. Swimsuit may come in handy for washing in the open. 200 ASA-speed film should be all you will need. Always carry more water than you think you'll ever need. Bottled water is available for purchase along many points on the trail, but it's sold by people from area villages and may not always be available, such as during bad weather.
I recommend bringing one of the new water-bottle purification systems (available from REI.com) to use to purify water from numerous streams along the way. You can also use other means of water purification (iodine tablets, drops etc., available from hiking stores in Cusco) but these usually impart a bad taste. Finally, carry a whistle to summon help, if necessary. These items along with the more general hiking/camping gear and food you'd take to your favorite national park, should have you prepared to hike the Inca Trail.