I've created a postcard template you can print and insert your contact information into, or open the PDF and modify it directly (don't ask me how to do this -- if you know, you know; if you don't, just print out the hard copy and paste in your information). Here's the template:
http://www.rwdigest.com/updatepostcard.pdf
Insert your contact information in the box provided (along with a special "reactivation offer" if you'd like) -- don't forget to include your return address on the right hand side of the postcard (see my sample for guidelines).
You can see how I've modified it (including making it a two-color postcard) at:
http://www.rwdigest.com/Bridgetupdatepostcard.pdf
You can also just use the FRONT side of the postcard, if you'd like, and create your own customized back side.
Photocopy it onto brightly-colored cardstock paper and cut to yield an 5-1/2 by 8-1/2 postcard (2 per page). These postcards require $.34 stamps ($.37 after June 30, 2002).
Or take it to your local quick printer and have it printed in color!
You can also use VistaPrint (50% Off All Postcards) to create custom printed postcards very inexpensively!