Beginning Writer's Answer Book by Jane Friedman
May I submit the same manuscript more than once to an editor (or agent) who has rejected it?
In the case of magazine articles, if the editor tells you he's rejecting it because he's overbought at the time, or he's recently bought something similar, you might have a chance at a later date. Also, watch the magazine's masthead. If an editor leaves, the new one might have different taste, and you might be able to sell him your manuscript or your idea. But use your best judgment before resubmitting. Try to ascertain if the editor was simply being polite, or if your manuscript or idea might really stand a chance at a later date.
In the case of book editors and literary agents, once you've been rejected, you've likely blown your chances with that person on that particular project—unless the editor/agent instructs you to revise and resubmit. If there's no invitation to resubmit, then you shouldn't keep trying, even if you do revise the work later on. While your revised work may indeed merit another look, most editors/agents do not want to see it again.
Beginning writers often look for ways around this advice, especially if they realize the original rejection was due to an unprofessional query letter or unpolished manuscript. Is it possible to change the book title, use a pen name, or alter details to disguise the re-submission? Give it a try if you like, but realize that agents and editors have a sharp eye and long memories. That's why it's so important that you query and send your work only when you're absolutely ready. In book publishing, it's touch to get a second chance on the same manuscript. Make it the best it can be before sending it out.