These are tips shared at a writing conference. You may read the full article [here].
1. Don't send your full manuscript off the get go. Only send what's requested in the particular agent's guidelines. (They almost always can be found online.)
2. Don't respond to a rejection letter or ask "why." Just move on.
3. Don't be rude or disrespectful—rejection isn't personal, it's just part of the business.
4. Don't submit to one agent at a time. The industry is slow so it's unrealistic for agents to assume you haven't sent it to other people (unless an agent asks for an exclusive read).
5. Don't resend a query or manuscript a day or two later with a note, "I found a mistake in my proposal and fixed it." Once it's sent, it's sent.