But even harder than querying is knowing what to do when you get something other than a form rejection in response.
It might be a simple request for a full, which is great! He/She liked it, really liked it!
But agents are often reluctant to sign an author off that full that you send them. They want "Revisions." Many people request those revisions before offering rep. I do because writing is a very different skill set than revising, and I want to see your revising muscles in action before signing you up.
More and more, agents are asking for these revisions on an exclusive basis because, naturally, we don't want all our hard editorial work wasted if you abandon us, cold and alone, for someone else...with our carefully thought out edits.
But be careful. Recognize that the agent is taking a voluntary risk by doing that editorial work without officially signing you. You aren't beholden to him/her, and you don't have to stop everything else because they gave you notes. If someone asks for an exclusive but you're still awaiting word from Dream Agent, it's OK to say no to that exclusive.You are only required to be transparent and professional, so if someone else asks for revisions too, you should let Agent 1 know about it, and tell Agent 2 where you are with Agent 1.
If the revisions conflict, well, that's the risk you took in querying multiple agents at the same time (which I support).
If the agent likes your work enough, they'll carry on even if you won't give them an exclusive. If they say nevermind, well, you were waiting for Dream Agent anyway.