Mountain bike cleats are typically fixed to your shoe using a 2-bolt system. Experimenting with the position of the cleat on your shoe will help you find the best fit for your style of pedalling.
With manufacturers like Crank Brothers, you can also adjust the amount of 'float' you have. This is how much you can twist your foot before unclipping from the pedal.
Finally, different pedal shapes will suit different riders. DHers tend to prefer flat pedals which can offer better 'feel' through your shoe, while XC riders tend to prefer slimmed down pedals which weigh less and offer a more concentrated contact point.