Mostly taken from this Reddit thread.

• A tennis ball container fits nicely in your water bottle cage, and will keep things dry. you can store you cell phone, arm warmers, wallet, etc in the the tennis ball container.
• I keep a small velcro tie (10cm or so) somewhere on the handlebars – I use it as a parking brake which lets you lean a bike up against a wall even if you are on a slope. Was very useful on a heavily loaded tour, and also useful when doing any sort of field service where you need the bike upright but don’t want it to roll away on you.
• I keep a secret $50 in my handlebar for emergency.
• have a friend take a photo of me standing behind my bike, then print out two copies, write my name, bike serial number, etc. on each photo. I roll up the photos and put one in my seat tube, one in my handlebars. If my bike is ever stolen, I can demonstrate ownership at the pawn shop, for the police, etc. And I keep updated records on each bike with every component, identifying marks and everything I can think of that makes the bike unique.
• If you see someone with a flat give them your tube, that one always comes back around.
• If your toes/feet get cold a lot…I read this in an article somewhere I can’t remember: before a ride/run/trail run, turn your socks inside out with your hand in them and sprinkle cayenne pepper powder around the toe area lightly (or wherever your feet get cold/numb). turn the sock right side out and carefully put on foot. I shit you not…this works for me every time. cayenne pepper is a vaso-dilator internally AND externally. it brings blood to the the skin’s surface. i’ve run through watery/snowy/frozen trails and ridden in shitty-cold temps and this solution works!