I think Freakonomics has taught a lot of individuals to be wary of "Buyer's Agents" and asked a really pointed question that bears answering: "how can someone who is representing my interests, really represent my interest when my purchase/loan essentially pays their commission, and the more I spend, the more I make, and some homes pay that representative more than others?" It's a great question, but in the context of a real estate transaction, where every property is different, and as NAR advertises, every market is different, a buyer's agent really can represent your best interests.
The first rule of equity building in real estate is that money is made on the buy. That's a great litmus test question to your agent: "how will you help me make money on the buy?" I would contend that a lot has been said about money being made on the sale, but that's not really the case. Every piece of real estate ha a variable price and a seller will not negotiate much off of their price if a buyer has a contingency holding up their closing, even in a bad seller's market. But that same seller will negotiate a lot for a different buyer that can close quickly, if that seller needs to get out. That's an example, a very profitable one, of how an agent actually represents the best interest of a buyer.
In Colorado we are a little different with transaction brokerage and buyer agency... to discuss that would probably muddy the waters for buyers and sellers in the other 49 states. But as Oliver Frascona likes to teach in continuing ed courses on the subject, there is a fundamental difference between these two types of agents, and as a consumer, you need to know this anywhere: a TB talks only about the property, an agent talks about more than the property (the people) and thus the motivation. So as a buyer, if you have a buyer's agent (or the reverse a seller's agent if you're selling), ask that agent to role play what they would do in a negotiation. There really aren't tricks, there are subtle questions. The best one as a buyer is back to my previous example: ask the other agent when the seller needs to close. If that gets given up, well, you as buyer now have power to accommodate that seller, and that power usually results in you getting terms, price, the copper pot rack, the plasma in the living room, whatever.
The last little bit I would suggest is to really make sure that agent is in tune with your problems and needs. If you didn't have a direct need, you wouldn't need an assistant (or advocate) who did this professionally. If someone is really listening to you, not just tracking with you, but confirming with you by re-stating and pursuing agreement as to what your needs/goals and desires are in the transaction, you can now begin to trust your gut because you have proof that the person is on-board with you.
Best of luck.