Introduction
Talk a lot about your agency history
Assume you know more than the client
Forget the client's budget constraints
Be oblivious to the client's other agencies
Ignore the RFP
Staff the pitch with who is available
Conclusion
You know how you find out what other agencies work for the client you're pitching? You ask the client. The client is often more than willing to share who its current agency relationships are with, who services which aspects of the business, and where the openings are.
It is suicide to come in and recite to the client who it is working with without confirmed knowledge. It demonstrates lack of research and immediately positions the agency for failure. Individuals on the client end tend to be very loyal to their agencies. It is much easier for someone to listen to you when you are not stepping on their toes.
Find out who, at the client, is the chief contact for each agency, and then who is actually conducting your pitch. From that you should be able to customize the pitch so that you are positioned as a nice little puzzle piece for them. No one wants confrontation, and no one wants to feel that their agency relationship is threatened. Only from the inside will you be able to avoid encrouching on other territory.
Interactive pitches are often clouded by the looming presence of the traditional agency, with the interactive agency as second-class citizens. Don't feel bad. Interactive agencies used to be fourth-class citizens but are now viewed as essential to most client plans. Seek to understand the dynamics of the interactive side of a client's business.
Learn to play nice with the traditional agency. The traditional agency often sets the overall arch of creative positioning. This is the reason you are a multi-disciplinary agency and why you are often not invited to interactive pitches. It is not that you don't have competencies in interactive, it's that the client cannot send confusing signals to their traditional agency.
Bottom line: If you don't try and prove to the client that you can replace the other agencies not involved in the pitch, you are less likely to cause internal conflict in their decision-making process.
Next: Ignore the RFP