Here is a brief article from http://blog.alexa.com/improving-your-alexa-rank/ that can help you to improve your website ranking.

What is the Alexa Rank?
Your Alexa Rank is an estimate of how popular your site is relative to all other sites. The Alexa Rank answers the question: Compared to all other sites, how is my site doing? Note that since rank is a relative measure, your site’s rank is not only dependent on that site’s traffic, but also on changes to traffic on other sites.

How does Alexa get its data?
Alexa has a large traffic data panel consisting of millions of people all over the world. Based on the data from this sample, Alexa estimates the number of visitors to all sites on the Internet. It’s a complicated calculation that involves correcting for biases as well as identifying and discarding fake or spam traffic. Our data scientists are the best in the business and they make sure the data we present is free of noise and non-human data, so you can make the best decisions for your business.

How is the Alexa Rank calculated?
Every day, Alexa estimates the average daily visitors and pageviews to every site over the past 3 months. The site with the highest combination of visitors and pageviews over the past 3 months is ranked #1. The site with the least is ranked somewhere around 30 million. If no one in our measurement panel visited a site over the past 3 months there is no rank at all for that site.

for more detail please find out here http://blog.alexa.com/improving-your-alexa-rank/