For writing, the rules outlined here are very helpful:

    [1] http://www.critters.org/c/diplomacy.ht
    [2] http://www.critters.org/c/whathow.ht

Specifically:

    * Critique the story, not the writer.
    * Remember that it's your *opinion*, and phrase it as such.

So, for example, if your friend's story had terrible dialog, you might say, "I felt like the dialog was wooden. It seemed to me like Joe Maincharacter used a lot of clichés -- maybe you did this intentionally for effect, but I found it tedious after a while."

This, of course, assumes that your friend specifically asked for a critique when sending you the story.