Oh man I had a huge thing typed up and I accidentally hit the back button X_X Ahhhhhh.
Oh well now I can make this not a giant text wall.
1.)Watch movies that take place IN the era. These will give you a good look at fashion, and slice-of-life for the times (the first example in my mind is My Dog Skip lol, because I've watched it 9001 times) Here is a list of movies that take place in the decade, look at the synopsises and see if there might be something you would like to watch, I can't say I've even watched a good fraction of these:
http://en.wikipedia.org/wiki/Category:Films_set_in_the_1940s
I've heard REALLY good things about The Pacific. With all that went on with me this summer I didn't bother to watch it but now I wish I did, apparently this is really good.
2.) Watch movies FROM the 1940s. This will give you a good idea of what influenced pop culture at the time, as well as the "modern" movies that take place in modern times (as of that decade) will give you an even more realistic look at life and culture. The 40s were a time some of the biggest movies of all time came out. I'm sure you've atleast heard of Citizen Kane and Casablanca. You'll probably be surprised with how many films you've probably seen that are from the time:
http://www.digitaldreamdoor.com/pages/movie-pages/movie_40s.html
3.) Listen to songs from the decade. This is probably more important then movies. Radio was still huge and really influence style and pop culture, like it still does today. This was also where Frank Sinatra comes in, and the crooning style gets really popular.
4.) Sports. It really depends on what is going on in your story, but it was important so looking up atleast the hugest sports stories of the decade might give you further insight.
5.) SUPER Important: From a culture standpoint, the decade really can be divided in half. War-Life and Post-War life. Fashion and especially musics and movies had drastic changes after the war, as the culture moved back toward excess and extravagance. Be sure you pay attention depending on the year your story takes place.
6.) I know you're writing a novel, but catalogs (think Sears & Roebuck, Montgomery Ward, etc. google American Mail-Order Catalogs) are helpful for life's little details. Clothes, Accessories, Furniture, Stoves, Car accessories, Picnic wares, I mean every little thing you have in your house and that you could get from a department store. While you're not illustrating, it will give you an idea of what things people owned and used were like, and you can get a realistic picture in your head that can only help you be more realistic in your writing. There have been reproductions of catalogs made.
Though now I wonder if catalogs were really as big in the 40s as they had been before. I'm pretty sure they were still around, but I dunno if they were like they were in decades past (right now, I'm studying the 20s). I think more and more people were getting cars and living in or near big cities so driving into town to go buy stuff from the department store was becoming more and more commonplace.
Google is also your friend for history references :D