In reply to John Rushby:
I agree with John pretty much throughout. I was out paddling in Peru for 2 months (though did quite a bit of trekking to get to rivers and also did the Santa Cruz circuit on my tod) in 1996 and although it was a while ago I'd have thought things will have only got easier (and busier). I'd say if you were intending to head to Huaraz or Cusco, then just turn up and sort yourself out with local info there. Hire a donkey + driver if you need to as locally to the start of the trek as you can (usually dead easy if you ask around the local village) and just walk.
Some points I'd recommend :
THe Inca trail is very overrated in my opinion, though MIGHT just be worth it to look down on MP from the sun gate IF you get there nice and early (6.00?). A night spent in the village at the bottom of MP (Las Termas ?) would probably be worth it unless its now commercialised to hell.
Santa Cruz trek is pretty easy, and if you go by the guidebook times (or worse take a guided trip) you'll be spending too much time on it. I personally think its comfortable doing the round trip in 2 to 2.5 days (road to road). I did it in less (day and a half) but was on my own and so just kept walking.
You'll find all the info you could ever wish for in Huaraz. Just get a Cruz del Sur bus up ther from Lima (get the hell out as fast as possible - Miraflores is only acceptable because the rest of Lima is so bad) and sort yourself out when you get there. The SA Explorers Club provides a VERY welcome haven in LIma and is an excellent source of info.
The trekking in more remote parts would be very worthwhile. If you end up taking a bus to Cabanaconde (bus from Arequipa) to see the Colca, then I'd recommend going a bit further in to the Colca. Take anohter bus to Huambo (they ran every 2nd or 3rd day when I was there), see if you can get hold of Susanna Yappo who'll sort you out with some burros and a guide (you caould manage easily without these, but you might have where the path out of the village starts) and walk down into the depths of the Colca canyon to Hacienda Canco. Awesome awesome scenery. It'll make the Grand Canyon look like Cheddar gorge. I reckon a day down to Canco and a day to get back (along the same path, but still worth it), although we took nearly 2 days to get to the bottom (we paddled out so didn't come back)
THere are some great sites and trekking to be had in the Sacred Valley which won't get half the traffic that the ones in the Blanca will / do.
Dont bother with guided tours, unless you're heading to more remote areas. You just wont need them if you can get a trekking guide book (Bradt for example) and if you do decide to hire a guide then just hire a local one - or a donkey driver.
Most importantly of all, learn Spanish - if you dont already speak it now, of course. Even a smattering will help no end.