Maybe this is common knowledge, but it's a new idea to me. Just read it in a saltwater fishing magazine.
If you have a trailerable boat and remove the drain plug to drain the bilge, there's almost always some water that doesn't drain out. You can either leaving the water in there or, what I've been doing is using a wet/dry vac to remove it which is sort of a pain. What I read is that you can take about a 3/4 inch strip of chamois (either natural or synthentic). From the outside of the boat, push one end of the strip through the drain plug and into the boat. The strip has to be long enough to reach the lowest point of the bilge. Then leave the rest of the strip hanging out of the drain hole. The end that hangs out has to be lower than the lowest part of the bilge. Kind of like you need one end of a siphon hose lower than the other end. The chamois will wick the water out of the bilge and it will drip off the low end of the strip. I didn't have a chamois at my house last night, so I just tore off a strip off an old T-shirt. Within a few minutes, the strip was saturated and water was dripping off the end. I put a container under the strip to catch the water to see how much drained out overnight and it looks like the setup worked really well. If rain water gets into your bilge while your boat's on the trailer, just leave the strip in the drain hole and the water will automatically drain out.