how do you mean? you can buff them up on a buffing wheel and using buffing paste but the finish you get with aluminuim dulls in time due to weather etc.
if you have scratches in your mirrors you can use birtex with combined use of brasso etc (think iv spelt it right), or wadpol (aircraft spec glass cleaner) which will remove the scratches and polish up the aluminium at the same time.
you might not be able to get hold of wadpol as its aircraft spec, i used to have some but someone nicked it !
hope this helps