The humble postcard is one of the oldest methods of promotion that can still be very effective today. It is a form of promotion that can work very well for graphical based clothing designs and very personalized brands. To work, it must have a specific purpose such as to advertise an event or to direct the recipient to your website. A good well designed and thought out postcard is the difference between the card being pinned up or being trashed, so you will need to make it as interesting and visually stimulating as possible.
You will want to avoid creating an obvious advertisement by displaying your product range on the postcard like you might an advertisement. Although your postcard is an advertisement of sort, you will need to think from a different angle. A postcard works best if the recipient keeps and displays the card or posts it to a friend.
Try personalizing the card as much as possible. Limited edition prints can work very well and make the owner feel as though they are holding something special. You can also get creative by how you use your postcards. For example, as an alternative to compliment slips or a business card. You can even use the card to post out to your customers with a short thank you note written on the back.
The standard size of your postcard will depend on what country you are in. The US has various sized postcards with the standard being 6” x 4 - 1/4. Also note that the maximum size the USPS will deliver is a maximum size of 6” long X 4 - 1/4” high and the minimum size is 5” long X 3 1/2” high. They are also very specific about the printed reverse of the card which needs to be inline with the US mailing guide lines. In the UK and Europe a postcard is a standard A6 size, measuring 105 mm × 148 mm.
If designing your own postcards you need to account for a bleed in the document size. This is an area around the card used when cutting the cards so they do not get an undesired white margin around the card. In the US, you should allow for 0.25” (1/4) at each edge of the card. In the UK you will need a 3 mm bleed around the edges.

The United States Postal Service has certain guidelines for the layout of the reverse of your postcard. The top left must be kept clear for postage stamps. The area below that to the left side will contain the address with also a strip at the bottom of the card which must also be kept free of text and graphics for barcode and post office markings.
Download Adobe Indesign postcard template files. These templates contain the .indd documents with the correct postcard sizing which also include bleed marks. Please note that this template was made in version CS4.