One of the key things I learned when doing account management is that my team were't just the people I worked with (other account managers).
My team consisted of everyone who could either get in the way or help me to deliver excellent service to my clients. That included people who worked for my clients too!
Perhaps the most important way of influencing them was to seek to understand what they did, why they did it and how they affected my clients' experience. I also needed to understand how I could add value to them and make their lives easier. Better to give value first and then ask for a favour.
When you become a freelancer, and are often thrown into a team, this is even more crucial. Here's a useful blog post that explains a strategy for integrating effectively with a team quickly.
http://strat-talking.com/account-teams-vs-strategists/