Be truthful and use common sense, don t try to trick people because they ll see through it. Include an A5 endorsement letter from an authority figure have a photo of that person and use their letterhead if possible. That is use people who advocate your services to your best advantage. 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 .