Use the correct mailing address with the Department, Suite, Floor, one letter directional's and complete zip+4 whenever possible. Use upper case letters with no punctuation, except for a dash between the zip and the plus 4 (e.g. 87542-5401).
Use fixed pitch, evenly spaced characters rather than proportional fonts. (All characters take the same amount of space).
Align all characters to the left. Do not center.
Place the address no higher than 2 3/4 inches from the bottom of the envelope.
Eliminate extraneous print around, and especially to the left, of the address. Keep all space clear to the left, right, and below the address.
Be sure the address is fully visible in window envelopes and doesn't slip out of sight if jogged up, down or sideways. Extraneous print must not be visible through the window.
Never use red envelopes, it interferes with the red ink the Post Office uses to cancel the stamp.