If you want your EOs to last, you need to keep them in a cool dark place in dark glass bottles (Some companies ship in hard plastic bottles for larger amounts and expect you to transfer them). You don't want them to evaporate or oxidize, so sunlight and air are no-no's. If stored properly, they can last years.
In soapmaking, the best way to make EOs scent last is to blend them in the "trinity method": a base, middle, and top note. As mentioned above, some believe clays or salt help anchor the scent. Storing your soap in a cool dry place with the different scents separated (like in lined cardboard boxes) is a good idea also.