I work with a lot of third party vendors on behalf of a large US corporation, and when looking at new vendors, I value
1) previous experience, especially with others in my industry. bonus points if you've done work that accomplished for your clients what I myself am trying to do
2) flexible packages/service levels - sometimes i have things in house that i want to reuse, sometimes i need more end-to-end support. be flexible and willing to meet me where i'm at
3a) extremely professional communication style and visual appeal, especially for a creative agency. the smaller you are, the more important this is.
3b) familiarize yourself with corporate language and realities. the larger the company you're pursuing, the longer things will take to get approved/signed/scheduled, etc. prepare yourself for those lead times.
4) Re: normal procedure for communication - try to get as close to the right decision maker as possible the first time you reach out. Otherwise you risk getting bounced around/ignored. This is tough as it might not be obvious at first - the bigger the company, the harder this is to find.
hope that helps!