Why wet sand?
There a few good reasons put forward. It helps to keep dust to a minimum. It keeps the sandpaper cleaner so you don't risk scratching what you are sanding. Here I will explain my method of wet sanding. I discovered it almost accidentally and it has worked very well. Or at least, in the state I was in at the time I thought it worked well.
Open the refrigerator and take out your can of beer. This part is very important. Not only is it beer (!) but it is the source of your moisture for wet-sanding. By this I mean the beer should be nice and cold, it should also preferably be delicious and not alcohol-free. Condensation on the outside of the can is what you are looking for here.
Place the beer onto your sandpaper. You can leave it here for a short time while some of the condensation cascades down the can onto the paper.
Pick up your beer can, take a nice long drink, and then place it somewhere else. (but keep it close at hand in case you need some more.. moisture.) With the hand that held the beer can, pick up the part of the model you will be sanding. The moisture from the can has now been transferred to the plastic via your hand.
With your free hand pick up the sandpaper and start sanding. See.. wasn't that easy?
You will notice that the longer you do this method, the better you think you are becoming.
And there you have it.
Now for some bling!