Last year, I had the bottom fixed by a pro. He first did a gelcoat directly under the ski (the keel) and he buffed the entire below with a buffing machine so I do not deserve the merit for under the ski. The trick is not to beach it at all.
For the white section and other plastics I used:
http://www.meguiars.com/en/marine/products/m5016-boatrv-cleaner-wax-liquid-16oz/.
For vinyl and rubber, I used:
http://www.meguiars.com/en/marine/products/m5716-vinyl-rubber-cleaner-conditioner-16oz/.
I think the real secret is to protect it from the sun in summer (put the tarp on when not using it). I always keep that wax with me in the cargo bay with a couple rags so every week or so in summer, I apply a thin coat on it to keep it shiny and protected.
The previous owner kept it very clean on the outside but neglected a bit the engine but once I replaced the engine, this machine is very pleasant to have.
Since I had the gelcoat redone, I am always using a small anchor to keep it in the water wherever I go. I first bought a cheap anchor and did some modifications as a friend explained me how it works in the navy! First bought a "Kwik Tek A-5 PWC Fluke Anchor with Nylon Bag and Rope" (on sale everywhere). Then I removed the rope and replaced it with a real marine rope that does not float (sinking rope). The rope should be the same length as your ski (minimally). Then at the end of the rope, buy some chain (medium size, about same size as the rope) and attach it to the rope for the same length again as your ski (minimally).
My anchor kit is now fantastic and never gets displace when there is wind so I never beach my ski. Secret is that the total length of rope/chain is at least twice the length of your ski and that it sinks. This sinking and chain adding some weight effect prevents the anchor from drifting. I had a friend of mine last year who bought the same kit and just added chain to the cheap rope and his ski was always getting displaced while mine didn't move an inch!
This is how you will keep even an old ski in good shape. And in winter, if you can't have it in a garage, get it shrink wrap as the tarp is not enough.
Benji.