I find it's just like buying any other kind of clothes- brands all fit different and you just have to try them on. I have broad shoulders and big boobs and have some mens jerseys that I've altered, cut out the elastic and put darts in some for shape. More aero. Performance bike has a cheap house jersy that I like, very shaped and usually good colors. I like Primal wear mens jerseys with the elastic cut out, they're awfully loud but they're cut for the general population (relaxed fit?) Their womens stuff is so pretty I just drool over it but it doesn't fit. Darn.
Sometimes you just have to buy the "unisex" for the smart aleck statement, too.