There's a strange irony that the way to get water out of a boat trailer braking system is to use water, but that's exactly the way drum brakes get a longer life. Trailer flush components are standard on a number of 2011 models, and trailer flush kits are available at most marine stores for about $50. They are simple to use; just attach a garden hose to a nozzle on the trailer and turn the water on. Dirt, sand, and the always-feared saltwater are removed within minutes.
One trick used by a number of BoatUS members is to move the trailer forward a few feet while the flush kit is in use. This ensures the brakes are thoroughly rinsed. Drum brake design is such that it holds water, hence the need for a thorough flushing.
There has been a continuing discussion online and at boat ramps about installing a flush kit on a trailer with disc brakes. Many say it can't hurt and are happy to wash down the trailer after loading a boat in saltwater. Still, others contend the disc brake need only be rinsed with a garden hose in hand, rather than with a flush kit.
For general maintenance, it is a good idea to take a look at the waterlines when the brakes are being flushed to ensure there is no leaking.