I've found that sticking the connector end back into the trailer frame to keep it out of the weather helps quite a bit. I try to keep it in there except when I need to plug it in. To keep from losing it I have it come out, and a bungee is wrapped around it and the frame, then it sticks back inside the "metal box" of the framework.
My next problem has been grounding, but on all trailers now I end up running a separate ground wire around everywhere, not relying on the frame. This makes things work _much_ more reliably for me.
The truck side of the connector - I have one of the 5blade types, with a cover. And I keep the adapter w/wire inside the truck. This has been a much better method than what I did on prior vehicles. It looks something like this:
http://www.etrailer.com/Wiring/Hopkins/HM48470.html
I suppose something like this could be mounted on both the trailer and vehicle, keeping a connectorized cable in the vehicles (both!) to hook it up. (Both -- so that there's always a backup plan!)
I'm looking forward to connecting up that boat trailer and bringing her back up to the lake.