This article is from the misc.writing Writing FAQ, by Wendy Chatley Green email@example.com with numerous contributions by others.
Insert your manuscript into an envelope that is big
enough to hold the manuscript unfolded. (That is, 9x11 is
fine; standard business-sized 4 x 9 1/2 is not, except for
VERY short fiction and poetry.) With your manuscript,
include either a self-addressed stamped envelope (SASE) big
enough to hold the return manuscript, or a smaller SASE for
the publisher's reply, with a note that the manuscript need
not be returned. Attach adequate postage to both envelopes.
Exception to the SASE rule: if you're submitting a
work to a publisher in another country, consider sending a
disposable manuscript, an addressed reply envelope for the
publisher's response, and two International Reply Coupons,
available at the local Post Office.
Wait. Start writing something else. Re-query (BY
MAIL) after twice the named latency period (a.k.a. the
response time.) If the publisher doesn't reply after what
you consider a reasonable time, write a polite letter
withdrawing the manuscript from consideration and resubmit