First rule is: If you can hoist it off the trailer instead of launching at a ramp, pay the extra and do it. If you have a tube trailer, that is a killer. You won't ever get the salt out; and rust never sleeps.
Next, if you have any carpet in the boat, take it out (if possible).
If you want to be really cool, before you hit the salt, wipe down any metal parts with a rag soaked in WD-40. It will make clean up a lot easier. Then, beyond that, have a great time, and wash it with soap and water when you are done.
If you do have to launch at a ramp, they usually have a hose-off for the boat/trailer. Bring a hose. The sooner you get it all rinsed off the better.
Wish I was going.
-j