Originally Posted by FBinNY

Next time the BB is out, or sooner if you want to prevent issues, drill a 2mm or so weep hole in the lowest point at the middle of the BB shell. If you have a screwed in cable guide, you can remove the screw form time to time (or permanently if you use a mild adhesive on the guide) to drain the frame. You can also drill out the screw to make it serve as a weep hole.

Most of the mischief happens from water trapped inside the frame, rather than from road spray. Water is very claustrophobic, and if you don't give it a way out it'll seek revenge on you and your bike.