Know Before You Go
It is not possible to climb Huayna Picchu without a ticket for Machu Picchu. You need tickets for both. Walking sticks are not allowed, as they are prohibited throughout the whole Inca ruins. Also, don't bring big backpacks, as parts of the path lead through very narrow tunnels. There are nasty gnats inside Machu Picchu. Especially, on this long hike, it is recommended to bring bug repellant.
As detailed above, there are two time slots each day. As Machu Picchu is often hidden in heavy mists in the morning, it is better to buy tickets for the 10:00 am group. The ruins down below will be most busy at noon as well, so you will avoid the crowds at the same time.
As of 2017, stricter rules have been placed on visiting Machu Picchu to deal with the masses that visit every day. Visitors will not be allowed to enter without an authorized guide. There are already arrows to help control the flow of visitors but these will be more prominent. There are also changes to opening times for Machu Picchu, Huayna Picchu/Wayna Picchu, and Cerro Machu Picchu.
Additionally, to climb Huayna Picchu, there is an age limit of 12 years or older. The age limit is imposed as the previously mentioned trek has a lot of slippery stone steps up a steep trail that requires the use of all hands, and hair-raising drop-offs.