1. Identify Which of Your Concerns Are Reasonable. Not reasonable concerns: "My agent is not garnering me twenty-figure advances," "My agent can't make me feel better about myself," "My agent doesn't want to hear about my divorce," "My agent is not psychic and does not anticipate my every whim," "My agent can't make my editor stop sending me TOTALLY HORRIFYING COVER COMPS." Reasonable concerns: "My agent won't email me back. Like, ever," "My agent is demonstrably unenthused about my current project,"* "My agent doesn't seem to remember my name," "My agent is refusing to tell me whether my book is on submission or not."
2. Direct communication. DIRECT. COMMUNICATION. By which we do not mean "hostile." You should NEVER feel uncomfortable sending your agent a polite, friendly email to check in. POLITE. FRIENDLY. "Hi, I haven't heard from you in a while, just wanted to touch base," for example. If you are trying to get information about something specific, BE SPECIFIC. "I sent 'My Awesome Novel' _____ months ago, can you let me know when you might have a chance to read it?" If your agent has given you a time frame in which s/he said s/he'd get to your project, remind him or her. POLITELY. "You mentioned you'd try to get to 'My Awesome Novel' a few weeks ago; have you had a chance to take a look?" Until your agent gives you cause to assume otherwise, give him or her the benefit of the doubt. People really are that busy sometimes.
3. Maintain a Tone of Professionalism At All Times. Even if you are slowly realizing you have signed a contract with Satan, always be the grownup. It never hurts. Publishing is a tiny, tiny fishbowl.
4. Establish Your Concerns. You have sent your agent a Friendly Check-In to no avail; it is time to get down to business. At this point, it is completely appropriate for you to state, clearly and firmly AND POLITELY, what you need your agent to change about the situation. For example: "I am concerned that you are not responding to my emails. Can we set up a time to talk?" "I'm concerned that you haven't read the book I sent you six months ago. Can we set up a time to talk?" "I feel out of the loop about the status of my project on submission. Can we set up a time to talk?" DO YOU SEE A PATTERN HERE. Give your agent an opportunity to step up to the plate. Maybe s/he is dealing with Terrible Personal Drama, maybe s/he has walking pneumonia, maybe s/he has seven thousand projects going all at once, maybe all your emails are going to the spam filter. Things happen, especially when you are dealing with human beings. But at this point, your agent needs to a. respond to you, obviously, and b. respond to you in a way that satisfactorily addresses the issues you have brought up.
5. If All Else Fails: It is Time to Fire Your Agent. This part sucks. There is no way around it. But remember: THIS IS YOUR CAREER. IT IS YOUR CAREER. An agent doesn't need to be your best friend and confidant, but s/he absolutely should be your advocate. You got an agent once; you can do it again. It is terrifying and exhausting and no fun, but having a bad agent is worse than having no agent at all. BE BRAVE. REMEMBER. THIS IS YOUR CAREER. And take heart: people leave their agents all the time, for a variety of reasons--their agent is quitting the business, their agent is representing different kinds of work, their agent is spending their advance on methamphetamine. It is not abnormal for a writer to work with several agents before finding the right fit. You are not alone, and agents will not think it is weird that you may have had another agent previously. Promise.
You should know the terms of your contract (WHAT DO YOU MEAN YOU DO NOT KNOW THE TERMS OF YOUR CONTRACT. AUTHOR-FRIENDS. DO NOT MAKE US COME OVER THERE AND SMACK YOU); thirty days' notice is pretty standard, but some contracts might require you to give longer. The lovely Editorial Anonymous had a fabulous and hilarious post recently on how NOT to fire your agent; take it to heart! Don't do any of those things! They aren't nice! Again, remember: NO MATTER HOW AWFUL your Bad Agent story, a classy exit is the best revenge. Don't start querying new agents until your relationship with your current agent has officially ended; no reputable agent will sign you while you're still under contract with someone else anyway. If your former agent sent your book out, get a submissions list from him or her before you move on. Your agent should provide this information for you without hesitation; if s/he does not, tell him or her the Rejectionist is going to be REALLY PISSED ON YOUR BEHALF. When you start querying new agents, it's fine to mention your old one. BONUS POINTS: If you have listened to us, you'll be able to say "I recently parted ways with my former agent on friendly terms" and not be lying. SEE HOW NICE THAT SOUNDS.
*Okay, a caveat on this one: it is your agent's job to tell you if your Current Project is a. unsalable and/or b. doody. Both of these things happen to the best of us; not a big deal, just means you need to write a different book. By "unenthused" we mean: your agent is not responding to you about your current project, doesn't seem to have a plan for what to do with it, has been sitting on it for months, etc. If your agent is telling you your book is unsalable and/or doody, and you have a historically positive relationship with your agent, your project might in fact be unsalable and/or doody. Sorry.