Slip out the back, Jack
Under-communicate. Don't talk to her about any concerns you have; instead, the breakup should come as a total surprise to her. If you have luck on your side, you may let her know on the same day that she gets an offer for your manuscript from a publisher. For the coup de grace, hire another agent before you fire her.
Make a new plan, Stan
Over-communicate. Send the agent partial first drafts so that she can see your writing before it puts on its makeup in the morning. Send threats, rants, and complaints directly to your publisher, without telling your agent first, so that you look like an unstable mess and she looks like she has no idea what her authors are up to.
You don't need to be coy, Roy
Have little concept of personal boundaries. Call her on her cell phone, crying, in the middle of the night. Talk to her about every problem you've ever had. Let her know how dysfunctional your family relationships are, and then tell her she's like a sister to you.
Hop on the bus, Gus
Once she's sold several books for you, fire her. After all, now you know the editors, so what do you need an agent for? Alternatively, while she's still your agent, go behind her back and sell books without her, and without ever talking to her about it. She's just a stepping stone, so step on her.
Just drop off the key, Lee
After she's fired, ask that her name be removed from your finalized contract so that she won't get her share of the royalties. (It won't work, but go ahead and try it. She won't be upset at all.) Then go on discussion boards and say nasty things about her. A classy agent can't and won't do this in return, so you're safe saying pretty much anything.
I know my readers are too smart and too kind to do any of these things. But it's still useful to know that they happen; that these are the treacherous seas that agents have to navigate. As true as it is that there are not-so-good agents who serve their clients poorly, there are plenty of wonderful agents who get bitten badly for their trouble. I know the publishing industry can seem brutal, callous, and cruel. Just remember that you don't have to be.