I have compiled just a few tips that have kept me going over the years. They have got me out of a fix or just made MTBing a bit more comfortable. Please feel free to add yours to this. I'm sure there are loads you have discovered.
Keeping that Camebak bladder clean. Rinse it out and keep in the freezer. Stops the mould growing.
2. Take an empty toothpaste container to use if you get a ripped tyre. Dead easy and it works.
Don't buy one of those rubbish mini pumps which cost a fortune. A cheap Zefal is better. Small diameter and longer stroke mean higher tyre pressure with ease. Fits in the Camelbak too.
Take a spare pump valve and shraeder/presta adaptor. If the value break you've got a spare. If your friends only have presta you can use one of their tubes!
Always take a chain splitter. It has got me out of jail in some remote areas. Better than a 5 mile hike with a bike. Also take a piece of chain and one of those Sram connectors.
Keeping that mobile or MP3 dry in wet conditions is a pain. Go to a sailing or surf shop or sports shop like Decathlon and buy a waterproof bag. It's cheap, wears well and does the job.
There is nothing worse than riding with fogged up glasses when you are short sighted!! Even the best vented ones fog up on a wet winters day during a long hard climb. A good sports shop with a diver's section will stock anti-fog spray. Works a treat.
Always pack an energy gel. When I'm struggling one of these babies with double caffeine does the trick!
Save yourself a lot of money and time by keeping that chain clean and more importantly checking it for wear regularly using a chain checker. I have been running the same drive train for 2 years by changing the chain when it is worn. These devices cost a few £s but save you more.