Slightly dry or overbaked cake
This is one of the easiest fixes. First, trim the crust to remove any area that may be hard. Second, produce a quick simple syrup that can be brushed over the cake. Allow the cake to sit for awhile so the syrup can be absorb and the moisture returned.
What if your cake needs a little more help? Slice the cake into 3 or 4 thin layers and brush each layer with simple syrup or a thin layer of jam. Use a filling that has moisture such as mousse, fruit or curds. The moisture from the filling, jam and/or simple syrup will be absorbed by the crumb of the cake.
Really dry cake
Now it is time to change your approach and turn that dry cake into something else. The most common solution is to cut the cake up into cubes and create a layered trifle instead. The cake is great for absorbing the flavored liqueur or sherry used in trifles. The layered combination of cream, jam, fresh fruit and liquor soaked cake is a great way to restore moisture and your guests will have no idea that your cake was ever anything less than perfect.
If the cake is too far gone, turn it into crumbs. Cake crumbs can be used in rumball recipes, bear claw fillings and combined with graham cracker crumbs to produce pie or cheesecake crusts.