Portion divide it.

If it's just you, divide the cake into sixths or eighths, then cut those sections into your normal serving size. Keep one section out; freeze the rest (assuming your freezer is large enough to accommodate it).

It usually keeps well, and if you use the whole thing within a month or two, it doesn't usually become watery or rubbery.

There's a neighbourhood bakery that does a standard-size fudge cake with fudge frosting; it's one of those cakes that are to die for. But there's no way in hell I could finish it, healthfully, within a week. Dividing it into slices and freezing, allows me to enjoy it at a leisurely pace.