What can I do to fix/improve a dry cake?
No matter what kind of cake you've made, if it turns out too dry, you can moisten it with an appropriately flavored liqueur or syrup. Use a skewer to poke holes every inch and a half or so, then use a pastry brush to paint on the liqueur or syrup getting more into the holes. Coffee syrups come in sugar free varieties if you'd rather not add more sugar.

As far as what is happening, since it is happening with different recipes, I suspect your oven. Is it running hot? Use an oven thermometer to be sure that you are getting the temperature you want. Also, check cakes frequently in the last 10 minutes of baking, using the method recommended in the recipe to gauge the doneness. Usually, the toothpick test works. Stick a toothpick into the cake and remove. If there is no batter on the toothpick (a few moist crumbs are OK), then the cake is done. Cakes that are overdone will always be dry.