What is the best way to prevent skidding/fishtailing on wet pavement?
Being aware of your weight distribution can help a lot. You want to make sure you are not leaning forward and taking pressure off of the rear tire. 
You can also help keep your rear tire planted by giving a slight downward twist on the handlebars with your wrists. Kind of the opposite of a bunny hop. 
As far as braking technique goes, try to be "progressive" about it. Instead of just applying all the braking power at once, or applying a tiny bit of brakes and just waiting for the bike to run out of steam. Try squeezing the lever, and as you start to slow, apply more and more brakes. This will help you stop in a shorter distance without your losing traction. 


A note about skidding: Generally rear tire skids can be handled when they happen. A lot of times the best way to ride them out is to keep on the brake and slowly ease it back out. If you just let go of the brake immediately, the rear tire can snap back around and it is easy to lose control. Front tire skids are a little different. If your front tire starts to skid you do want to let go of the front brake immediately. Keeping your front tire rolling is very important to maintaining control of the bike.