Be positive on the Inca Trail and enjoy the scenary. It's hard work, but there are plenty of rest stops and no pressure to hurry up, everyone can take their own pace and it is ok if the group is quite spread out. Do some training beforehand to improve fitness and stamina.
There is a LOT of packing, unpacking and re-packing so be organised with your gear. Take two bags - a nice spacious suitcase for general use and then a soft carry all or the exodus kit bag that you can fold up and put inside the suitcase. The suitcase is fine to use in the cities and for the jungle and trek you can use the soft bag and leave everything else in the suitcase. Bring a suitcase with some space left in it as it is bound to get filled up! We had to unpack and repack our gear in the check in area of the airport at one point as our stuff was spread over 2 bags and we needed to fit them all into the suitcase again for the flight.
Beware - 7kg for the personal Inca Trail gear allowance does not go a long way! Most of us carried that almost much in our day packs too! The sleeping bags that some of us hired were very warm but also heavy so took up most of the weight allowance.
The porters are amazing and the meals on the Inca Trail (in fact the food over all) is a very high standard, we had meals whilst camping that were better than you might get in a restaurant!
Get several large dry bags - for both day packs and hold alls. No'one cared about wearing smelly, dirty clothes day after day but no'one wanted to wear wet clothes! It can pour with rain at any point in the trip and it is possible for bags to fall into rivers. Also have a plastic wallet/folder for passport, paper etc. You will get a lot of bits of paper so I took an A5 clear wallet with me to keep them safe.
Bring a Nalgene or Sigg/Karrimor water bottle as you can fill them up with hot water and use them as a hot water bottle at night.
Take one reasonably nice (but easy to wear/fold up) shirt or blouse as there are several opportunities to eat in a restaurant. I felt like a real scruff on the final night wearing the same casual shirt I'd worn for so much of the trip.
Take plenty of batteries for your camera if it takes AA/AAAs. If it is a rechargable one, bring a spare as there is no where to recharge on the trek (you can in the jungle for several hours each evening).
Be prepared for some energentic dancing as there will be a fiesta on Amantani Island! It is great fun and you will also dress in local clothing styles!
Bring anti travel-sickness pills if you need them as there are long bus and train rides.
Bring plenty of lip balm and moisturising cream as the Inca Trail and Lake Titicaca can really dry your skin out.
The trip notes say to bring dollars and change small amounts into sols along the way, but we found it much better to keep most of the currency in sols and only keep a smaller amount (about 30%) back left as dollars. Everywhere takes sols anyway. It is also good to have a lot of change and small notes in sols rather than big bills. A lot of public lavatories charge 1 sol per visit so keep a supply of coins!
I found a pair of good walking books, light trainers/tennis shoes and a pair of sandals good for the trip but in hindsight I would have taken light walking shoes instead of trainers (as well as good, worn in boots) and sandals. There are plenty of times such as day trips, bus rides, plane journey etc where you don't want to have to wear the heavy boots all the time but there is still a reasonable amount of walking around and tennis shoes had a bit of a thin sole.
if you forget anything for the trek there are plenty of good outdoor shops in Cusco which sell all the usual gear. The best thing I bought was a waterproof rucksack cover!
Shopping in Lima for gifts was very expensive (western prices) - try and buy your gifts at local markets instead.
Get a woolly Peruvian hat in Cusco for the Inca Trail , they are excellent and only about £2 from market vendors!
Take an small inflatable pillow if you need a pillow for the Inca Trail, or a pillow case that you can stuff with your fleece, spare clothes etc for sleeping on.
Bring a small (handsized) travel towel for the Inca Trail as they give you a bowl of warm water to wash in so you want something to dry with.
Be prepared for every type of climate - from 0 degrees to 30+ and high humidity! A good wind proof jacket and a light fleece are both good ideas, or a thick windproof fleece. I had a thick fleece but it would have been a good idea to take a micro fleece or thin zip up jacket as well for in the citiesor when it was not too cold. Get really good waterproofs - do the shower test if necessary! No point in getting an expensive goretex jacket if it lets the rain through coming down Dead Woman's Pass, as some of our group discovered!!
There is a LOT of tipping, not just on the Inca trail, but the drivers and local guides as well. So budget for that as the trip notes make out that you just tip the Inca trail team and main guide.
During the homestay on Lake Titicaca the reed boat ride, lunch and dinner on the first day are not included in the overall price and you need to pay in sols for these when you are there (5 sols for the boat ride and 15 per meal), it is not much but it used up most of the money I had put aside for local crafts.