Don't be fooled by promises and pricing that are simply to good to be true. A common trick for tour "companies" in Peru is to advertise the price but then somewhere during the tour you will need to pay for park entrances or meals or some other component that you thought was included. Check references.
Dress in layers in the Mountain regions of Peru. The day can be hot, clear and beautiful but as soon as the sun dips behind the clouds or mountains the temperature can drop dramatically. Even during the normal course of the day it is much better add or remove layers than to be stuck with a coat that is either to much on, or not enough off. I wear SPF long sleeves no matter how nice the day is and one of those goofy Lawrence of Arabia baseball-looking hats with the flap on the neck, while I am doing any outdoor days.
Remember you are somewhere between 8-12 thousand ft. and the sun is much more intense there. The problem is that the air is dry and most of the time the temperatures are mild so the temptation is to "bare the skin". You will get burnt without sunscreen. I have seen MANY lobster red tourists running around with zinc oxyde on their face so they don't miss the vacation, due to yesterdays sunburn.
I found out the hard way that those little black biting flies can ruin your day at Machu Picchu. I have been there over 25 times and only wore shorts the first time. The weather is mild (and I live in the Amazon Jungle) but either way I got eaten alive and those welts HURT. I would love to tell you that I figured out what season/temperature etc. they are active, but that would be a lie. Sometimes they are there and sometimes not.
Let someone knowledgeable (like me) help you with your tour schedules. There are some things that you simply will not think of until it is to late. Here is an example. If you go on the late train to Machu Picchu from Ollantaytambo, and stay all day in Machu Picchu and Agua Caliente, then take the late train and bus back to Cusco it is dark both ways on the train and you see NOTHING.....good thing you just paid extra for the "Vista Dome"...You know? the train with the windows in the roof so you can see the really awesome scenery.