That said, there's one very important thing I'd like you to consider when querying agents. Jessica Faust has written a very thoughtful and diplomatic post on the subject, and I think it's something that should be said directly (if it hasn't been already): you are not necessarily an agent's first priority.
Please understand that I don't mean to be rude when I say this. Please also understand that I don't mean this in the sense that you are not necessarily the best potential client to query a given agent (this may be true), or that said agent has other obligations, i.e. a family, that probably come before you (this may be true as well). What I'm saying is that in addition to you, one of thousands of potential clients, any given agent has several existing clients who need his or her help with all the things I blog about: co-op, discussing misleading BookScan numbers with publishers, understanding the terms of their contracts, and so on. I imagine most agents are absolutely exhausted by the work they put in soothing, cajoling, and haggling with editors, clients, and publishers, and frankly, I'm impressed that some of them have as much time for potential clients as they do. Again, this is no slight to you; you are not unimportant. You just aren't as important to the agent as the agent is to you. At least, not yet.
There are ways to improve your odds, however. In addition to paying attention to the basics, making sure you're ready to query, following the guidelines and writing a kick-ass book, you could take a chance on a younger agent with fewer/no existing clients, or try querying in the summer, when the industry is running a little slower. If you read agent blogs (which you should), try to avoid querying when you know an agent will be on vacation, possibly somewhere without reliable Internet access.
At the end of the day, the publishing industry is just that: an industry, and it depends on sales. Does it depend on sales to the exclusion of all else? No. But agents, publishing houses, and book stores all have to turn a profit in order to remain in business, and unless that changes, the majority of attention will always have to be paid to the individuals publishing now, and not those who may be involved at some undetermined point in the future.