As in any campaign, there are many things to consider when sending your query letter to literary agents.
It is also important to note that you should not query two or more agents at the same agency at the same time. Send it to one agent at that company. If that agent rejects it, you are free to send it to another agent at that same agency.
You should send your query to multiple agents/agencies simultaneously. Giving an agent an “exclusive” while you wait to hear back from her merely indicates that you don’t know how the publishing world works. You don’t need to mention that you’ve sent the query to other agents. They will assume that you have.
I recommend making a spreadsheet that includes the following: Agent Name, Agency, Date Sent, Date Responded, and Response (including any notes the agent may give regarding why it was rejected). Often the note you’ll make is simply “no request made for my book.” When an agent explains why she didn’t offer representation, I copy the email and paste it into this spreadsheet. I do not paraphrase because I don’t want to lie to myself. I worry that a month later, I might claim that the rejection was for some other reason that I like better. You want honesty. Brutal honesty might be the best kind.
You will get a lot of rejection. In fact, I just got rejected last week! But it is okay! No worries. Thankfully, The Dark Knight Returns Part 1 had just come out on DVD and it cheered me up immensely. COMPLETELY UNRELATED SIDE-NOTE: the above is an animated version of the celebrated Batman Graphic Novel by Frank Miller and Klaus Janson. If you like Batman, you should read it.