First you need to make sure that your pivot point is at the center. If not, just press ctrl G to group your object to itself. Then delete half of your model and go to edit>duplicate>options box. Next, put a minus sign next to the appropriate X Y Z column in the scale section (This all depends on what plane you are trying to duplicate across). Finally, decide whather you want to make a regular duplicate, or an instance, which will allow you to work on one half and the other side will update at the same time.

I think that's right but I am still at work on a night shift and so not at my computer or completely with it.:zzz: