We've been there, in fact we were literary agents for a little while in the early 2000s, and had a lot of clients to work with. The insane amount of submissions we got ranged from piss-poor to brilliant, but in most cases we thought people were just going willy-nilly about getting an agent. They didn't research who we were, what we did, what genre we liked, etc.
Allow us to provide some very good advice when you're ready for the big leagues, and need an agent.
Research. NOT AGENTS...but BOOKS.
Why? Target and research closely those books out there that have been done already that in some ways are SIMILAR to the book or books you've written and READ them. Go with really popular, successful books too. Who wrote them? Find out who the authors are of these books you've found that are like yours, and THEN find out who their agents are. Why?
Because this is what agents really LOVE to see in your cover letter:
"Dear Agent:
"Waiting for the Dragons" was, in my opinion, probably author Benhamin Brown's best work yet. (explain why)
My name is _____ and I'd appreciate it very much if you'd like to take a look at my work, "_______" which also involves dragons trying to find a new paradise (brief summary and it's okay to spoil)...yadda yadda..."
See what you're doing here? You're commending the AGENT for taking on such a good writer, and letting them know that, hey guess what, you've got a book that could be just as successful for them - maybe even more so.
Getting the picture?
Good.
Go get that agent.