Jeff,
I believe a lot of people (including myself) like the postcard route. Of course, some do prefer letters. The post card in my opinion sort of forces them to read it or at least grabs their attention.
During the start of the foreclosure process I'm sure they are receiving all sorts of mail and the last thing they want is to open another letter from someone they don't know.
I am working on a couple of short sale deals as I type; getting the leads aren't the hard part. Getting them to work with you, collecting all the documents, and actually negotiating with the banks is what can be tough.
As far as getting leads, definitely do some sort of direct mail to NODs. Also, if you aren't afraid to put out bandit signs...those will get your phone ringing as well. But be aware of the sign police too....oh and you should get a mentor. Or someone who's at least done a deal or two who can hold your hand practically and walk you through it all. There are some on here that have tons of experience with shorts. My mentor sure is doing a hell of a job with me...
Good luck!