Your question seems to hint at some deeper misunderstanding. Forgive me if I'm wrong.
A variable is just that - it can vary at the whim of the programmer. You don't need to clear a variable before using it for something else. It is down to you, however, to write your code to ensure that it isn't still being used for one task before putting it to use by another.
There is a case for 'clearing' variables which reference Objects as soon as you can, however (as sedj shows above). This might allow the garbage collector to recover the memory used by the object sooner if nothing else references it.
Tim
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"Your morbid fear of losing,
destroys the lives you're using." - Ozzy