I got an e-mail the other day from an author - an unpublished one - asking for advice about who took slush (unsolicited) subs besides Baen. His comment - "I don't want to send my manuscript to an agent - I earn little enough without paying them." showed me just what misinformation is still out there.
The first absolute and inviolate rule of publishing (and there are publishers, agents, distributors and retailers all keen to break it) is that money flows toward the author. That is the purpose of copyright. It's there to give rights to the author, so that the creative arts might flourish to society's benefit (when I was young, we 'ad dinosaurs, and also some politicians who took the long, wide view...) An agency charging a reading fee or recommending a book doctor is not a literary agency. They're scam artists preying on people's hopes and dreams, to be regarded with the same disgust as vanity publishers (which is not the same as self-publishing).
There are plenty of agents who have set out what they do and don't do. I'm sure Amanda can point you at some of the sites - I know she has in the past. They're useful to read. What, sadly, there isn't is an equally frank set out of what authors and publishers think good agents do and don't do. The problem is that publishers have gradually changed the job of agents, and made authors very dependent on them, which is in fact the inverse of their original role -- once upon a time agents took a very small percentage of the author's income to see the author could get on with it and write with the minimum hassle. You only got an agent when you were well established and at least potentially very high-earning. The agent did all those things that so many of us are really bad at, from negotiation to being a social secretary and psychologist. At a certain level of success (a very high one) this is still an agent's role. But there are few clients that can support this (call it level A). Most of us will never be... and somewhere down the line agents started taking in lesser names and noobs. It's fairly obvious that at this level the agent has one principal purpose: to sell your book and to hopefully raise you to the next level of desirability as a client. Mostly when you read agent XYZ rant about not being client's shrink or diaper-changer that is the level they refer to - which is where most of us are, and in reality will probably stay. As publishers have abrogated the task of filtering unsolicited subs the position of agents to noobs has become the inverse of agent's original client/author relationship - you need them, they don't need you. None-the-less it's not as unequal and dependent a relationship as some agents portray. It's quite common for authors to 'move up' to other more powerful agents. However - inevitably - this situation has led to a few agents behaving like abusive husbands, with 'wifey-author' trapped in a relationship that destroys them. Seriously, just like in those reationships, the best way is not get into them in the first place, and if you are in one, painful as it may be, get out with minimum damage if possible. There are some things as an author you need in your relationship with your agent and there are some mistakes not to make. Ask me. I've made most of them. Firstly you are a person of (potential) value. Your agent is not your shrink or social secretary. They will not necessarily sell a book quickly and easily - and being 'needy' and bugging them every 10 minutes is not helpful. But you are not their secretary either. If months pass and they never communicate, and do not reply to e-mails (which are not every 10 minute needys) then really, they're not interested.
Be VERY wary about being taken on by an agent at the commendation of a better-known author-friend. The agent needs to _like_ your work to sell it. I appreciate being turned down by Sarah's agent -- it improved my respect for her. Ideally your agent (if you are published) should have read and liked your work before you approach them. If they see you as a potential cash cow, but really like a different style of book, don't. They'll be bad at selling what you want to write, and be very likely to try and push you into stuff you don't want to do. And this is not a relationship of equals - agents are often wrong (so are authors), but don't try and tell them this. Just go elsewhere.
Finally, your agent - at this level - is essentially a salesman. Do not remind them of this fact (some of them are rather twitchy about their importance), but when you go through the list of their do's and don'ts that's really all that is left. Their primary job is to sell your work. It's not quick or easy, and you need to make it as easy as possible. That means there is one author responsibility that I have never seen any agent mention: you need to give them stuff to sell. Just as getting married is not the end point in a relationship... or to put it more bluntly - If you want have a baby just because you had sex once doesn't mean you're going to get pregnant. It might never happen, but it probably won't happen if after nine months nothing has occurred... and you'd better try again. you have to keep up new proposals and books, even if you really want to sell the older ones. And if nothing happens for say 4 years of mutual effort... well accept you're not fertile together and go elsewhere.