I just got back from the Classic Inca Trail with Intrepid Travel and wanted to share what really helped me deal with altitude sickness, since I was pretty worried about it beforehand.

Before the trek:

Arrived in Cusco 3 days early to acclimatize (this was crucial!)
Took it super easy the first 2 days - just walked around slowly, no strenuous activities
Drank coca tea at the hotel every morning and evening
Stayed really hydrated - like 4-5 liters of water per day

What I packed:

Diamox (altitude sickness medication) - got it prescribed before I left
Ibuprofen for headaches
Electrolyte tablets for my water bottle
Coca leaves (bought in Cusco - way cheaper than at home)

During the trek:

Started taking Diamox 2 days before we began hiking
Our guide was amazing at setting a "pole pole" (slowly slowly) pace
Forced myself to drink water even when I wasn't thirsty
The coca leaves actually did help with the headaches

What surprised me:

Day 2 (Dead Woman's Pass at 4,215m) was tough but manageable with proper pacing
Sleeping at altitude was harder than hiking - bring earplugs!
The guides carry oxygen tanks just in case (never needed it but good to know)

My advice:

Don't skip the acclimatization days in Cusco - I saw people who flew in the day before really struggling
Listen to your body - there's no shame in taking breaks
The porters and guides know their stuff - trust them when they tell you to slow down

Anyone else doing this trek soon? Happy to answer questions! The whole experience was incredible and totally worth the preparation.

Peru: Trek the Inca Trail 4 Days by Intrepid Travel