Safety Checks before you set off
Before you set off, make sure your bike is in working order. If your bike is working correctly you will have a much more enjoyable cycle and it will be safer. Mountain bikes take a lot of punishment along the trail. They are asked to handle all sorts of terrain, handle responsively and then they are pushed to maybe 50kph downhill and expected to stop as immediately as soon as you misjudge a corner.
Personally I prefer to check my bike after a cycle. Usually I will power-wash the bike, wax it and lubricate it. I give it a quick check over to make sure it is ready for the next trip. That way I have no surprises on the day of the cycle. The basic maintenance for a bike is to clean and lubricate it. Keeping derailleurs clean and free of dirt and oiled will hugely increase their lifespan. As a general rule, you oil fast moving parts and grease slow moving parts. Usually the better the components are on the bike, the less maintenance they require.
Wheels
Make sure the spokes are tight, that none are broken and that the quick release levers are tight on the forks and chain stays. Make sure there are no buckles or cracks in the rim. Wheels perform a lot better now than they used to. Most wheels are built and trued by machine but more expensive wheels are generally built by hand and are trued to a higher degree of accuracy. Ensure that the wheels spin freely and that the hubs are not binding up. If you spin a wheel and it slows down rapidly, then more than likely it has damaged bearings or the cones (cups that screw onto the axle) are too tight. If the wheels are used in this condition for long they will soon end up in the bin.
Tyres
Check for rips and splits. Replace them if they are bad or patch them from the inside. Check that there is nothing sharp in your tyres that will cause a puncture. Remove it if there is. Make sure there is the correct amount of air in your tyres and repair the puncture if they are going flat between rides. Match your tyres to the type of riding you do. A tyre that clogs full of mud will not perform properly. Similarly, a narrow slick tyre is of no use on a forest trail.
Chain
It is quite common for chains to snap when mountain biking. To reduce the risk of this on the trail you can make some checks before you leave. What you are looking for is a click from a certain point on the chain when you pedal backwards. This click may be caused by a stiff link, and this can be due to either a bent side plate on the chain and/or a rivet out of position. This situation will result in the chain snapping as soon as it becomes under pressure, like riding up a hill. If your chain is jumping (usually around the jockey wheels on the derailleur) then replace the problem link or the chain. Chains actually have to be replaced regularly because they stretch. When they stretch, they cause wear on the sprockets because they don’t fit the teeth properly and then the sprockets will also need replacing. Your chain needs to be matched to your sprockets and chainsets. If you have a 9 speed cassette for example then you need a 9 speed chain. Ask your local bike shop for advice.
Brakes
Check that your brakes are working correctly and not rubbing against the wheel or disk. If you have hydraulic brakes (cables filled with fluid instead of wire) make sure you have no leaks and don’t pull the brakes when the disk is not in place because the pads will close and you will not be able to fit the wheel as a result. If you do pull them by accident and find yourself unable to get the wheel onto the bike then use a wedge of some sort to open the pads by force at the caliper. A metal tyre lever or screw driver will do. They will self-adjust when you do get the wheel back on. Check to see if the disk is warped. Disks can either become warped by impact or by excessive heat during breaking. If they are warped they can sometimes be straightened by hand or they may need replacing.
If you have cable brakes, then make sure the cables are in good condition. One indicator of damaged cables is the brake caliper not opening easily after the brakes have been applied. If this is happening then you may be able to improve the operation by greasing the inner cable. Replace your cables if they are frayed or kinked but if they have been on for a year or more you might as well replace them.
Gears
Check that your gears are working correctly. Make sure that the chain does not fall off when you change from gear to gear. If it does fall off then the range on the derailleur probably needs adjustment on either the front or rear derailleur, depending on where the chain is falling off. The range adjustment is done by adjusting two screws on the derailleur itself. These are usually marked H & L for high gears and low gears. So if for example you find that when you change down to the small ring on the chainset, that the chain falls off then you may need to tighten the L screw on the front derailleur to reduce the range.
On a back derailleur H will adjust the range on the smaller sprockets and L on the larger sprockets. On the front derailleur H will adjust the larger chainring and L will adjust the smaller chainring