O-rings

Save lots of money by replacing O-rings instead of replacing entire items, like flashlights. This place has replacement O-rings for bargain prices:
http://www.oringsusa.com/catalog/.

I order for different gear at one time so that the shipping cost works in my favor. I admit that it took me a few weeks of trial and error before I figured out how to size O-rings accurately enough for my gear. Also, here's a good summary about materials:
http://www.marcorubber.com/materialguide.htm

Lubricate your O-rings to make them last longer and to keep their water resistance. I use a silicone grease.
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If you're reading this, it's too late.