This topic is probably the most widely misunderstood or confused from all the weddings that I’ve covered. From my experience, a lot of couples and their bridal parties often are not aware of the traditional wedding etiquette in wearing a buttonhole (for groomsmen) or a corsage (typically for the mothers of bride and groom).
What I have learned is that, traditionally:
Of course there are always variations to these traditions. For example, if men have military medals, these go on the left chest and the buttonhole would then be worn on the right; I've also been to a lot of weddings where the corsage was worn by the ladies the "right way up" - as long as everyone matches, I guess it doesn't matter too much!
At the end of the day though, it's your wedding, and you should do as you please - treat this as a general guide.