The main problem with making a photo look old is everyone thinks there should be tons of grain in it.  The only problem with that is that if you look at a picture from 1940 (assuming it is in focus) they really are quite sharp with grain being almost entirely non-existent or extremely small.  Try this out.  Instead of grain, take the image and grayscale it, bump up the contrast a ton and a half, then put some scratches and dirt specks on it and see what you get.



http://www.b17.org/images/timephoto08.jpg

http://upload.wikimedia.org/wikipedia/commons/d/dc/B-17_bw_left_inflight.jpg