From my understanding, the two best non-postcard direct mail approaches are:
1) "lumpy mail" with green diamonds on the outer edge, oversized in 9 1/2 x 11 envelopes.. "lumpy" meaning there's some promo item inside (cheap something), to grab attention
and
2) the handwritten ones with no return address, which have inside, a yellow sticky lift note, eg "Thought you'd like to see this" - J ...
Along with a "tear sheet", eg an advertorial pitch that looks like someone tore it out of a magazine
(very old approach, but still effective)
For postcards, I'd simply have a big text on it, saying
"go to
www.mydomain.com/FreeVideo.mpg to grab a "7 Power Golf Tips...." video you've got to see, if you want to slash your golf score by 15 strokes the next time you hit the fairway... " etc and a benefit-laden stock photo, eg shot of golf ball going into hole, or a scorecard with "94" on it or whatever.. (using golf as an example)
Ken