Put a few upside down and backwards. If you're very unfortunate and have to pay taxes, use a two or three party check. On top of paying with a three party check pay one of the dollars you owe in cash. When an extractor recieves cash, no matter how small an amount, he has to take it to a special desk and fill out a few nasty forms. Write a little letter of appreciation.
Any letter received has to be read and stamped regardless of what it is or what it is written on.
Write your letter on something unshapely and unconventional. Like on the back of a Kroger sack. Big envelopes have to be torn and sorted differntly than regular business size ones. An added bonus to the big envelope is that they take priority over other mail, so the workers can hurry up and deal with your mess. If you send 2 checks they'll have to staple your unsightly envelope to your half destroyed form.