there is thread in this forum regarding this. here it is http://forums.digitalpoint.com/showthread.php?t=1840032
yes this is little difficult to do that you only have to gain high traffic as soon as your alexa will be drop day by day....please see my signature website i have personal experience...
you know , Alexa rank is relevant to traffics , so by increasing the traffics , you can get a better ALexa rank !