When pitching your book, either in person at a conference or through a query letter, it is common to compare your book to something similar. This is a double edged sword. Agents want to know what your book is like and the easiest way is to compare it to something similar. On the other hand, if they've recently sold something like it, or if they feel the market is flooded with books like it, they are going to pass. So how do you overcome this dilemma?
You must answer both questions: What book is your novel like, and more importantly, how is it different than anything out there right now? The second part is the part that will hook the agent in and make them want to read more. It's what will hopefully make them think they can sell it despite the similarities to what's currently on the market or about to hit it.
Think long and hard about what makes your book different. If it is too similar to something on the market, you may need to alter it a bit. But if you have just the right combination of tapping into something that's hot, and introducing something different at the same time, you'll have agents excited about the project and hopefully lining up!
Here's a link to part 2 of What Agents Hate on Guide to Literary Agents: http://www.guidetoliteraryagents.com/blog/What+Agents+Hate+Part+II+Author+101+Series.aspx