The newest bikes have 27 or 30 gears.

The actual usable number of gears is around 14 because some combinations are almost identical.

 

Let the chain run straight as much as possible, not diagonally.

 

Anticipate course changes and shift gears smaller.

 

Always shift at the front first, the chain has to travel longer at the front. It is easier to shift at the rear when the chain is under tension. The less force there is on the chain, the easier you shift. So never shift at full power on the pedals.

 

When downhill, put the chain on the center chainring to prevent the chain from flying off due to the shocks.