Using disguise is mainly a business to business approach but it works for business to consumer mailings too. Business to business promotional mailings are marked out in that they don't appear to have been handled individually. This can be indicated by an envelope pre-printed with a postage mark, and use of a window or address labels to show an address. Try one or more of the following to give the envelope a more individual look. Use stamps. They have to have been put on by hand.
So it must be individual. There's no more Executive List obvious an indicator than a pre-printed stamp impression. When a preprinted impression is used in conjunction with a window envelope it practically screams advertising so don't do it. Hand write the address on the envelope. This is probably only viable for higher value items but will certainly get more attention this way. It doesn't have to be copperplate, it just needs to look as though it's been handled individually. Use an unusual material for the envelope.
Say vellum or a textured paper. In tests conducted by a paper merchant, this alone increased responses significantly. The additional cost for the special paper was more than compensated for by the additional revenue generated by the mailing. Again, do anything to make your mailing appear more individual and personal to the recipient. Probably the most important element is the use of stamps. This on it's own can be enough to get you under the recipient's radar. Secondly, use persuasive tactics.