A couple notes before you submit:-
1) Submit to one literary agent at a time in a given agency (no matter how big, I learned this the hard way). If they pass on the query letter, then you can usually submit to another unless they specifically state that a “no” from one agent is a “no” from all agents at that agency. Some say that literary agents pass queries on to internally other agents – in my experience, I have not seen this at all.
2) Choose your agents carefully. Someone who has published say Stephenie Meyer is not really going to take on your vampire romance novel if she’s already got a successful author running with that.
3) Make sure they represent your genre whether it’s fantasy, romance, young adult, etc. You don’t want to waste someone’s time with your novel if they only represent non-fiction.
4) Track your submissions in your excel spreadsheet to stay on top of timing, agents, agencies, responses, follow-ups, etc. It’s good to keep track of agents requesting partials (3 chapters, 50 pages, etc.) and/or full manuscripts.
5) Make sure your prospective agent accepts email queries. Stick to their specific submission guidelines – some prefer email, some prefer snail mail, and some use forms on their websites.
Check out Agent Query’s page on Submitting to Literary Agents for additional tips and guidelines.