So with this in mind direct mail should aim at being more personal if any company wishes to increase the probability of anyone opening the direct mail in the first place. A personalized envelope will stand a higher chance to be opened than any other.
Avoid using plain white envelopes, make them colorful and give them a personal touch, make it look as if you have created that envelope specifically for your potential client. When you place your sender's address try and make that personal looking as well and not just a stamped series of labels. If you add a personal name to the company's on the return address, people will be curious to know who this person is that is sending them the mail and will actually open the envelope and you're half way there.
Although it's true that automated stamps are great for saving money, they are not so for attracting attention. If you personally stamp your direct mail, this will give it a personal touch, and people will feel you have spent time on them and will also appreciate the business behind this touch. A company who spends time and money on potential customers, is, to the eyes of the latter reliable and different to the others.
When you write the address go for a handwriting font as opposed to the classic Times New Roman most companies use. Yet again this will render your direct mail more personal and similar to any personal mail the person may be receiving from a friend or relative.
By using these subtle but effective methods your direct mailing will stand up amongst the many other standard and impersonal advertising letters, that will just be left unopened or disregarded and thrown away on the spot. People will more likely open a letter that seems directly addressed to them, with the above personal touches and once they've opened it your message will have got through.