As of August 26th, 2015 I have terminated my agreement for representation by agent Robin Rue of Writers House. Since we've worked together since 1998 this was a very tough decision, but our contract prevents me from indy publishing, or taking on certain projects my freelance clients would like me to do. Also, as I am comfortable getting jobs and vetting contracts on my own now, I will not be acquiring a new agent.
There are lots of articles online about how to terminate an agent; Miss Snark has the simplest and most direct one here. It's not something you want to do on the phone, in my opinion, but if you're on good personal terms with your agent and want to have a dialogue about your reasons before you send the official letter you certainly can. No one likes being fired, but agents have to deal with it probably more often than any of us. Be professional, be honest but keep your emotions out of it, and you'll probably receive the same in return.
It can be depressing, especially when you've had only one agent as long as I have, but once you're ready to move on there's no point in dithering. It's best to make a clean break and get on with your career.
As for me, this was the first step to take toward indy publishing, so now I can move forward with my plans to pursue that. If all goes well with budgeting, and finding the right people to work with, I should be able to start publishing books under my byline in a few months.