What can I do to fix/improve a dry cake? So a lot of times a cake is really dry, whether the recipe author intentionally does that or if I did something wrong I'm not sure (maybe it's how it's meant to be but it doesn't suit my tastes).

What can I do after the cake has been cooked and finished to make it not so dry?

And similarly, what can I do before it's been cooked to make it not so dry? I figure adding butter would help, but I'd rather not add more fat / calories, so I'd prefer an alternative.
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.