Thanks for the nods guys, glad folks are getting some value out of the info. There are a lot of nuances and details not discussed here and I think that split testing (or, better yet, multi-variate) is a bit more intense when doing Display. Something to keep in mind.
One other thing that is not marketing related but is absolutely crucial is the ability to negotiate. Even if you don't think you have good negotiating skills now, know that like any other skill, it is something that can be learned.
Learn the terminology as well. Simply talking the talk can get 20% off your rates instantly.
The big publishers have more inventory to sell than they know what to do with and EVERYONE is going to quote you their worst bids in the beginning to make up for all the money they're losing on that inventory. Talk to a lot of Networks/Publishers and play them off one another. Tell them you want Agency Rates not standard Rate Card.
You'll probably get asked what your budget is on a monthly basis. Don't give em' numbers, tell them that if their traffic delivers acceptable ROI, you have an unlimited budget. Tell them you're a performance marketer and track to the penny. If the traffic doesn't convert or if the traffic is not optimized on their end, it'll just be a one and done. Tell them you need to test the traffic first and want to do a $5k or $10k test buy. Let them know that after a successful test you'll talk to them about a larger buy. With your eCPM in hand, you know exactly what you can pay. If they won't sell you targeted or guaranteed traffic at that rate, try remnant or tell them, thanks, cya. You'll find quite often that they'll be calling you in 24-48 hours accepting your rate. It's also a lot easier to get all your clauses accepted at that point.
Won't happen all the time - especially where there are a lot of Branders doing big buys but just wanted to illustrate that negotiating is key and with non-transparent traffic, it can be difficult to discover and pay the true value of a particular traffic source.