I thought it would be a good thing to have a thread where people post and discuss little tricks they came up, during the course of their cycling and bike-repair experience.
One trick I have: I noticed that, no matter how good the rubber insulation on the battery lid of the cyclecomputer, in the Finnish wintery rains, at least a tiny little bit of water will make its way in, and cause a little bit of condensate inside. That might be just a cosmetic problem, or it could drain your battery's life, depending on the amount and place of condensation. This is even much more pronounced with bike lights, where the water can find much larger "holes" on the lid (much longer edge), and the insulation is usually worse than with cyclecomputers.
My solution is to put a little bit of lithium grease on the strategic places. Lithium grease is hydro-repellant and very stable. Together with the existing insulation, it will provide nearly 100% security against water infiltrations. Try to avoid getting it on the contacts with the cradle, even though it's not critical.