Most of it's been said in the reviews, but do also check the Arrivals Gate forum on here, because there's lots of good advice there that I missed before I went.
Unless you're very young and healthy, take trekking poles, they really help with all the steps even if you don't particularly have knee problems. Don't do this trip if you do have knee problems though, you will really struggle (one of our group had real problems) and it then slows the rest of the group down.
Also take your own down sleeping bag, it weighs so much less and the weight limit is quite restrictive otherwise. Advice varies but your sleeping bag is not going to get wet (the tents are very waterproof) and there's no other reason why you wouldn't want to use down. I took a 3 season sleeping bag with a silk liner. On cold nights they fill your water bottle with hot water to put in your bag - excellent idea, and worth (as I did) taking a metal bottle just for that purpose, even if you drink out of a Camelbak during the day. I used a Thermarest and my own down sleeping bag, total weight 1 1/2 kilos. Some people were already up to 4-5 kilos with just those two things.
You don't need many clothes, just a couple of tops, a couple of pairs of trousers, some decent thermals and a few changes of underwear. After our first day in Cusco was absolutely freezing, I couldn't believe I would ever be in shorts, but the weather changes dramatically from hour to hour and also in different locations. Do take trousers that zip off into shorts, much more practical. Also do take good wicking base layers (not cotton tshirts), you won't regret it. Also plenty of layers that you can put on, especially at night. In the day I wore mainly tshirt and shorts, plus at times a light fleece and a fleece jacket and poncho (and long trousers), in the evenings I wore 2 thermal base layer tops, a fleece jumper and a light down jacket, and thermal leggings under light trousers, and was only just warm enough. If you wear shorts during the day, use mosquito repellent even if you don't see any mosquitoes...I never saw any but they found my ankles all right! I was nervous about the "toilet tent" but it was not nearly as bad as some reviews have indicated - actually far nicer than the public loos and they never smelt at all!
Invest a few soles in a plastic poncho (sold everywhere) to go over or wear instead of a rainjacket - it can get very wet and they are very effective at keeping the worst of the rain off, especially for your backpack too (use a waterproof backpack cover too). You really don't want to get everything wet on the trek as it'll never dry out.
Most important thing of all: even if the weather is not hot, drink lots of water and take rehydration tablets too. They say 2 litres a day - I'd recommend 3-4 litres. Don't underestimate how much the altitude will dehydrate you - I thought I was drinking a lot and still suffered severe dehydration, to the extent that I collapsed a couple of times and started to lose consciousness.
Do the Rainforest Extension trip too (I think there's now a proper combined trip with the Inca Trail). But be aware, there are lots of creepy crawlies, snakes, piranhas and so on, so if you're afraid of these, the trip is not for you! Even more important to take good technical wicking base layers - you need long sleeves but it is extremely hot and humid so you will sweat buckets. Take a good book as there isn't much to do in the evenings and during the free time. Binoculars are really essential if you want to see wildlife (and there is plenty to see) and invest in lots of plastic ziplock bags for anything you don't want to get wet, and take a bin bags to line the kitbag they give you, as it's not waterproof. My only criticism of the guides in the Rainforest is that they were informative, but nowhere near the league of Exodus guides. They were a little distant and didn't give the impression of looking after you, just of instructing you. For example, our guide didn't ask about any medical problems, carried no first aid kit on a full day's walk in the jungle, and wasn't even interested when I fell and dislocated my shoulder, not even asking me how I was on the following days. Not a problem as I am pretty self-sufficient, but not what I was expecting, having been used to the caring friendly and organised nature of the Exodus guides.