Be aware that when you're in Huaraz, just about everyone will try to sell you a trip with the 'best guide service in town'. You may meet some random person on the street who seems like they are just trying to help you out, but they are probably trying to get a commission by referring you to a service. There are tons of agencies, and guides work for multiple agencies. I wouldn't necessarily just go with the first thing that looks good. Be sure to know EXACTLY what is included in the price (transport, food, porters, # days, fees, etc), how many people are going, who is your guide, etc.
Skyline Adventure School is a top-notch outfit and they will customize any trip you want. They aren't the bargain basement of Huaraz guides. If you're sole criteria is a cheap service, you have lots of options. If you want great, friendly people with top-notch service, look into Skyline. There are several on SP who have used them. If you don't want a guide, they can also arrange logistics, as can (I think) some others .
Oh yeah, and the page Kevin was referring to was this one: http://www.summitpost.org/logistical-ce ... -peru.html
If any other Huaraz veterans have anything to add to that page, let me know. It'd be good to have a diversity of opinion.