The weather in Cusco can change drastically during the day and also throughout the year, according to the season. But there are two things you can count on for sure once you are in Cusco: from January to December if you are under the sun, you will be hot—usually during the day, under the sun, you will be hot—usually during the day.
Having said that, and to be more specific, the weather in Cusco and the weather in Machu Picchu are characterized by two seasons: rainy season (December - April) and dry season (May - November).
During the day
During the night
Waterproof clothing
Umbrella
Closed and resistant footwear
During the day
During the night
Warm clothing
Water
Insect repellent
Lenses with UV protection
Sunblock
Remember to be prepared for altitude sickness also known as "soroche". Machu Picchu is a high altitude area. For more information read "Altitude in Machu Picchu".