As your company creates more web hits and volume, at what point do you have the leverage to approach large companies for their advertising dollar?
I don`t know the key to success, but the key to failure is trying to please everybody.
In my opinion the tipping point is different for each site. Depending on your volume of traffic and the focus of your site it could be day 1, or day 1001. The best thing you can do it start asking earlier. Most companies will give you the guidelines, or criteria they look for when it comes to advertising on a site. Also be prepared to offer `deals` initially to bring in an advertiser. Many advertisers are wary of venturing out to unproven sites without some hard business metrics, or some type of short term / minimal commitment.
Also don`t over promise on what kinds of results your advertisers will get. I typically advise my clients to not promise much of anything other than "about" how often there ads will show, how they will be placed etc. Many people promise ClickThru rates or X impressions in a day/week/month. The click thru promise is a bad one to make unless you are doing something nefarious to guarantee those clicks. Impressions are a better promise to make but again only promise what you know you can deliver.
Keep in mind that sometimes ad`s do poorly because the advertiser didn`t take the time to create an add which will "work" on your site and "talk" to your audience. They will try to blame you and sometimes you`ll just have to suck it up and work out a deal (don`t ever give back money, but offer a reduced rate on a new campaign and suggest some changes to there ads).
Unless you get into a program ad management etc is a touch sell. I`d start a new post here where people can suggest ad providers and talk about there experience with them etc.