If you already have a contract, there's no longer very much that an agent can do for you. In fact, in most cases I'd recommend hiring a contracts lawyer instead. Flat fee, you pay less, and you're less likely to get scalded by a nasty contract clause if you've got someone who actually understands law reading the contract. I'm not slamming agents with that statement - they just don't have any real training with legal issues, and over the last couple of years we've seen many major publishers introducing clauses to many contracts which are extremely dangerous to the writer, even career ending in some cases.
I hear this a lot - and I'm not saying its not a viable opinion but I would like to offer up a few things.
There are varying degrees of contracts. When your agent is from a "big firm" and you are dealing with a "big six" then they already have negotiated some language that will be to an author's benefit. Even if you already have a contract in hand, as long as you haven't signed it the agent (or lawyer) will get a better one than you would get on your own.
My agent actually IS a lawyer so they are not mutually exclusive - sometimes you can get both.
A one-time fee sounds like a no-brainer but it could be higher than a commission arrangement. If the offer is $5,000 than the agent would get $750 (assuming you don't earn out which occurs 20% of the time) and at $350 an hour (the going rate for many IP lawyers you can eat through that quite a bit.
My agent is always on the look out for "other" revenue sources for my work whereas a lawyer is just a one-time transaction. I've made more than $200,000 from additional sales closed by my agent including: foreign language sales (8 countries and counting), audio rights, book club, and she has even landed me one of the largest media agents (movie / television) of which she'll get a cut if a deal is made but more than worth it if it comes in.
Over the last couple of years we've seen many major publishers introducing clauses to many contracts which are extremely dangerous to the writer, even career ending in some cases.
This is indeed true and I had to fight tooth and nail to get alterations on this front (and almost didn't sign because of it). To be honest they are pretty easy to spot but very hard to get the publisher to "budge from". As I mentioned my agent is a lawyer but she also hired (on her dime not mine) an IP lawyer just because he had done many more deals than her (800 - 1000). They both told me the same thing, "This is standard in all contracts, the publisher will not alter this clause." They both told me I would have to live with it or walk away. Well I couldn't live with it and would have walked away but after 4 months and with a release deadline approaching I finally got a contract that was agreeable to both myself and my publisher. Bottom line...you are your advocates (lawyer or agent) can advise and "attempt" to get alterations and in the end only you can decide if you can live with what is eventually presented for signing.
Again, I'm not disputing what Kevin (and others) have said in this debate I just want to shed some other perspectives on it as well.