Be Prepared
Misti is not the most difficult trek in Peru. But you should not treat it lightly. Elevation may require the most fit hiker to turn back due to altitude sickness.
You must be prepared mentally & physically. You must haul the right gear <checklist>.
You must be informed. Do your homework. Take notes from or print sections of this website. Do not hike Colca without a copy of Lonely Planet Trekking in the Central Andes.
First Aid Kit
Prepare for the worst! You need a far better First Aid kit than usual on Misti as you may find no one else to care for you.
Someone in your group should be qualified to administer First Aid. This is a serious responsibility.