Ooh, the 1940s! I suppose a truly authentic 1940s wedding would be rather austere, considering the rationing a bride would have to plan around. But the music would be amazing — big band, a young Frank Sinatra, Duke Ellington, Ella Fitzgerald, Louis Armstrong. If you wanted, you could hire a swing instructor to give your guests a quick, 10 minute lesson on the basic steps right at the start of the dancing at your reception.
This is also one of my favorite historical eras for fashion. It’s where we really see women take on menswear and dramatic shoulders and necklines. I love this dress from Elizabeth Fillmore — it’s very 40s http://www.elizabethfillmorebridal.com/gowns/spring-2013/arielle.html. Stephanie James also does dresses inspired from the 1940s and 50s. http://stephaniejamescouture.com/ If you wanted a more A-line look, you could also use Queen Elizabeth’s dress as a form of inspiration. http://37.media.tumblr.com/tumblr_lkpu5tzvaV1qiyye8o1_500.jpg It also seems like there are a ton of vintage wedding dresses available on etsy and ebay so that would be a great place to look, too.
I love the Nancy Drew idea the PP mentioned. You could also have each table named after a major Hollywood starlet from the 1940s, like Rita Hayworth, Ingrid Bergman, Humphrey Bogart, Katherine Hepburn, Clarke Gable, etc. You could probably find vintage posters from films like Casablanca, The Best Years of Our Lives, It’s a Wonderful Life, etc. to decorate.
I’m not certain, but I would imagine most bouquets from the 1940s would be in classic white, ivory or off-white flowers, nothing too dramatic.
For other decorations, maybe if you could find vintage radios , those would be a neat touch. Disney also became a major pop culture force in the 40s, so maybe vintage figurines of Donald Duck, Mickey Mouse, Dumbo, etc.
Good luck with your planning!