Do research each agent you send a manuscript to.
Don’t send out a mass email to as many agents as you can.
Do spell the agent's name correctly.
Don’t address your query with the heading, To Whom It May Concern or Dear Agent.
Do check your query for grammar mistakes.
Don’t send out an unedited letter.
Do follow submission guidelines.
Don’t use any crazy gimmicks.
Do tell the agent why you are sending your manuscript to them.
Don’t tell them that it is the next best seller.
Do study other successful query letters.
Don’t be arrogant.
Do put some personality into your letter.
Don’t be unprofessional.