What goes in the trail bag?

This area is often debated and there is no real answer as it depends on the person, the location and what you plan on doing, although you never really plan to fall off or snap a spoke. Over the years I have built up items to carry with me, First is the backpack, I use Camelbak either the SCUDO or The DON They both carry 3L of water, I easily get through this much water so if necessary bring bottled water or a second bladder.
CamelBak Scudo
CamelBak The Don

The DON has a greater carry capacity and is designed to hold armour too so I use this more for DH as there is a space for the full face helmet and you can attach some of the arm/knee pads to it. It depends on how much you will be carrying, some like to pack light. The important bit is carry enough water especially in armour as you will use more from sweating. On a single ride of about 5 - 8 hours I will go through 6L easily but I often see other riders with 500ml for the day and dehydrated muscles is one of the worst things for you as this greatly effects performance.

 Every rider should carry a decent multi tool, a tool with a variety of allen keys, tyre levers, link extractor etc, a decent relatively cheap tool for this is the Specialized Evo:
Specialized EMT Comp

This tool is well made and has everything you will need to fix or adjust out on the trails. Granted it is not a substitute for a bike stand and full tool box but can prevent you from having to abandon the day.

 Any rider can tell you that at some point you will get a flat, depending on the terrain they may be more frequent. Most people carry puncture repair kits, but some of the cheap ones are not really that good as it all comes down to how good the patches are and the range of patches available to you. When it comes to patches people think of thorns, nails, glass etc all of which puncture in similar styles, however rocks can offer different types from slices or blow outs needing different patches, I usually go with the Rema range:
Rema Tip Top Medium

 Of course anyone that has been riding long enough knows that there are times that no matter how many patches you have the tyre still goes down and it is always best to keep a spare tube too. I usually keep a light, compact tube in my bag as a back up.Well the next logical item would be something to pump it up with. Pump technology has changed a great deal in the time that I have been riding meaning they no longer have to be cumbersome items that you need to strap to the frame. Micro pumps and CO2 pumps have changed this giving you about 30 PSi or more per canister while making them light and small enough to fit in a pouch. I opted for the Genuine Innovations Second Wind: This pump is tiny and has presta and schrader valves, it also doubles up as a hand pump.
Genuine Innovations Second Wind

A few years a go I started to pack a first aid kit as had come off a few time times and thought I may need one. Since then I have used this more on other people than myself but with exception to water it is the most used item in my pack and I have come to realize no one seems to pack, but you WILL come off, one of these comes in useful. I found a travel first aid kit from boots will do most things that does not require paramedics.

It is always best to keep a split link in a pouch in case the chain goes which I have seen happen. I degrease my chain regularly and replace it roughly every year to reduce the chance of it snapping. This will be the typical items I pack when out on the bike, this will change depending on terrain and the type of riding you will be doing but it covers the essentials.