Gather testimonials and use referees. Window envelopes look like bills so don't use them, however it does guarantee that they ll be opened. Labels are impersonal. Address printed directly onto an envelope gives a much more personal touch. Handwritten envelopes have by far the best impact because they are the most personalised of all addressing methods.
Postage stamps are much more personalised than franking or any other postal mark.
Type letters in an easy to read font then sign in blue ink. If you use gimmicks, use them to ensure the envelope gets opened and not binned . If you include a gimmick indicating the time you ll save, use it to increase the response rate. The prospect can also feel the bulk of the egg timer inside the letter and they will open it. Make sure your letters are personalised.