Put together eleven snail-mail queries today, plus sent out three more e-mail queries, for my currently active manuscript Unscheduled Inertia. This is actually the second sci-fi novel I've written, the first of which is currently a gibbering mass of entangled plot threads that requires database experience to even follow. I've got an initial database of 30 agents to query, which means I'm about half done.
If you're in the same situation, avoid re-inventing the wheel. Kat Dancing has done a lot of the hard work for us. (thanks Kat!) It should be emphasized that Kat's list should just be the start of your research. Research each agent, by looking at the publisher's marketplace member's page, the agent's blog if they have one, and Amazon. (Put the agent's name in quotes in the search dialogue box of the books section & you'll get all the books that have them in the acknowledgements). You want to know if they actually represent books like yours, and if they do how to submit to them. I have no idea why an agent would claim that they represent sci-fi when every single client is a romance writer, but it's more common than you'd think.
The process of querying is slightly more pleasant than removing your own fingernails with pliers. It's both repetitive and precise, with just enough variation between each letter that you can't let your attention slip. You need to send a lot of query letters to have even a chance, but no two agents are the same. Some want five sample pages, some want ten, some want three chapters, some want some combination of the previous and a plot summary as well. Sometimes you know something about the agent that you can include in the letter, or you have met them at an event, in which case you want to include that.
And then there's the fact that one little slip-up can screw you completely. Mistype the agent's name, have a typo in the cover letter template that you have re-used for all 25 agents you're querying, forget a cover letter, mis-address the letter...the pitfalls are endless.
And after all this work you know that the odds are reasonable that your only reward will be steady trickle of rejection letters, with at best a few requests for sample chapters.
But I keep it up. You be the hare, I'll be the turtle, and I'll be in Scotland afore ye, or something like that. Good night.