Look at, and use, these buses:
Ormeño and Cruz del Sur:
And look what I wrote on the Peru transportation page-there is a tip on buses and one on airplanes.
Info January 2009
I was in Ayacucho. Time came to leave. In the evening got on a nice bus bound for Pisco, on the coast. I wanted to return to Pisco for a couple of nights...primarily to take some pictures that I had missed on the first trip.
Leaving Ayacucho, we had to wind over the mountains in the night. Half way over the mountains we came across another bus that was broke down. We stopped and they all got on....grandmothers, children, men with huge bags of coco leaves and bundles of blankets and everything. The clucking chickens and dead animals were either put on the roof or under in the baggage compartments.
So we had a bus that holds like fifty with about 100 people in it. And we continued over the mountains.
So when you want to get out....when you reach your destination....you are supposed to go to the front and bang and hammer and yell at the drivers compartment until he stops. But when we reached my stop, I had fallen asleep and I did not wake up until we had reached Lima.
From abroad, by plane.
From Peru itself, at least from the coast, there are plenty of coaches, some of them being really luxury ones. If you have time, why not?
I did myslef the trip from Lima to Santiago in February 1998 (company Ormeños). 60 hours!! A once in a lifetime experience. I won't do it a second time. For sure, you will know your neighbours and learn why Peruvian go to work in Cile (if you can speak Spanish) and they might invite you home to drink some Pisco.
Plenty of cheap buses, but it looks like each bus belongs to a different company. So, do not expect any plan or schedule o bus stop, just ask people in the street or at your hotel. And ask the driver where you have to get out.
Otherwise, taxis are cheap. You just have to negociate the price before getting in. Minimum fare in 2000: 3 soles. For less than 10 soles you could already go to a different neighbourhood.
You can also rent a car, although I would consider doing it only if you want to go out of Lima. Driving in Lima is quite crazy.
There is a new website in peru to compare bus prices and itineraries: You should check it out : www.busportal.pe