What additives, if any, should be added and how often?
It depends on how the vehicle is used. If you are running the vehicle and burning through a tank of gas every week, fuel addatives are probably unneccisary on a regular basis. If you aren't running the vehicle often, an additive like Sta-Bil can help keep the fuel fresh while in the tank.
As far as additives go, some additives work better than others. BG products have a very good reputations, but are a little expensive (and can only be purchased at an authorized dealer which is usually a garage - not at a Walmart or such). If used correctly, Seafoam is a good product as well. It is also a lot cheaper an can be readily purchased at a Walmart or Autozone. Over time, varnish does build up inside the intake tract. These types of products can help to clean them up.
None and unneccessary would be the answer to your question. The fuel companies spend good money on their fuel formulations and pretty well cover all that is needed. There is though a vibrant after market that will try to convince you other wise. Over many years I have yet to come across an after market product that really does do what it says on the tin in way of improvement. USA and European governments exert stringent controls on fuel supplys and suppliers.