I'm crafting an alternate history to support a story I'm writing. When I sat down to play "what if?" I chose the approach of listing the aspects of dieselpunk (and steampunk) that I most liked and the sensibilities ascribed to them. Then I went about trying to craft justification for them (ie, why did the alterna-tech not come into the forefront in the main timestream, and what could change to make it so?). I don't know if this is the wisest way to create a logical existence for alternative reality, but it seems to be working so far, if in fits and starts.
I'm lurking all over the forums here at dp.org and alternatehistory.com. I waffle back and forth between intimidation of the sheer amount of historical detail there is to process, and the burning need to damn the torpedoes and set Zeppelins to warp. I've always believed that the best research one does for fiction writing is something that one absorbs, internalizes, and expresses unconsciously. Facts and figures and interesting tidbits always seem to be lampshaded, unintentionally or not, in fiction I read. The real feel of a book comes from its unconscious tone--the sense that the author is comfortable and familiar in the subject. It's just a little more difficult to feel comfortable in an alternate timeline. :)
Signing off for now from the USS Valkyrie, high in the skies somewhere over a 1930's Europe that never saw the end of World War I