Postage stamps are a great finishing touch for the numerous bits of correspondence that pop up in Lovecraft's stories and "Call of Cthulhu" adventures. A huge chunk of "The Whisperer in Darkness" is a simple exchange of letters, while the climax of "The Shadow Out of Time" is set into motion by a note from Australia arriving in Arkham. Some of the most effective published scenarios start with the characters receiving a letter or package in the mail, reinforcing the postman's role as a harbinger of impending.
For the ultimate in prop realism the best approach is to use actual stamps from the 20's and 30's. That might seem over the top, but it's easy to do and suprisingly affordable. Ebay is filled with examples of nearly new period stamps that can be had for under a dollar, and cancelled stamps are available for just pennies.
Another option is faux postage- stamps that look authentic under casual examination. Anyone with a computer and a printer can whip out reasonable facsimiles of stamps, but as with any official paper there's a balancing act between making them look real without crossing the line into forgery. Here's my first crack at it:
Click through for the high-res version- a 300 DPI JPG sized at .75 by .85 inches, one of the most common United States stamp sizes. While there's definitely room for improvement, I can't add the one thing that will instantly increase it's believability by an order of magnitude- a perforated edge. Those distinctive scalloped serrations are a vital part of the sensory identity of postage stamps. Luckily, the explosion of scrapbooking as a hobby means you'll find a variety of perforating scissors and cutting wheels in your nearest craft store.