Stamped, self addressed envelopes in case the jargon isn't second nature yet.
All agents require them if you query by snail mail.
The first time someone ambushed me with "what do you do with all those stamps" I was a little taken aback. I thought it was a joke. I was at a writing conference in front of 400 people so I said "I use them at the liquor store instead of cash of course."
Then someone else asked.
And then it showed up on the comments pile here.
I know I don't keep them. I know I answer every query letter I get that has one. I also know not all of you are getting them back.
First. Size matters. #10 SASE means a #10 sized envelope. It's the size a manuscript page fits in when folded in thirds. It's NOT a small envelope like you'd use in the offeratory plate at church or an odd sized envelope like you get for a wedding invitation. #10 is on the box of envelopes when you buy them. Look for it.
Second. Your address. Write it in ink. No return address labels from Easter Seals in place of the address. NO LABELS at all. If your handwriting sucks, learn to print envelopes on your printer. Pay your spouse to write them. Labels can peel off or get torn.
Third. Don't write the address in ink that smears. My office is infested with coffee gremlins, water bottles, and the like. The mailbox on the corner isn't exempt from being rained on.
Fourth. Put a stamp on it. You'd be surprised.
Fifth. If you don't hear back, write again. If you don't hear back again, you've just saved yourself from being represented by people who are not well organized. Not an expensive lesson at 34cents a pop. (oops. postage is 37 cents a pop as pointed out by a Snarkling in the comments section. All in all though, still a bargain rate.)
Sixth. If you don't hear back from ANYONE or more than 50% of the people you mailed to, the problem is on YOUR end.
Agents in the business of committing fraud do it in much more lucrative ways than stealing first class stamps. They ask you for reading fees. Or production costs. Or loans.