(Please note that this is different from "Things that make me laugh." These things do not make me laugh, or piss me off. I'm neutral about them, but it doesn't mean you should do them)
A lot of query suggestion websites recommend personalizing the letter by letting the agent know how you heard about them. This ONLY applies to people who know the agent through a special methods, like a convention meeting or they know a client of the agent.
Many people have, in their desperation and desire to listen to advice, written things like, "I found your agency in Writer's Digest." Well, of course you did. You had to get our name and address from somewhere and that's the first place people look besides the internet. It's like saying, "I learned about your agency from a phone book."
That's what Writer's Digest is: a phone book. It contains names, addresses, and other contact info the agent wishes to be made public, like an email address or a phone number. It also contains what genres the agent handles, and then a bunch of statistics that I'm pretty sure they just make up. I don't know of a single agent who can say off-hand what percentage of their client list is new authors, and I don't know of a single agent who's willing to put in the time to figure it out for Writer's Digest. Writer's Digest is full of filler, bad advice, and listings for unscrupulous agencies and writing contests. And, there's a decent chance that by the time it comes out, the information is invalid. Agents are always moving around, joining up or breaking off from larger agencies, or going out of business. Things happen very fast in New York City. I honestly would go to AgentQuery.com.
And if you do go there, don't mention it in your query letter.