The problem is that many sponsors take Alexa rank religion. They believe in it as it was holy. I also have the same problem one a few sites that the unique visitor number is great, but Alexa rank is comparably low.
However most of the websites having an Alexa of over 1 million are mostly crap (mostly, not always ).
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There's actually a "black hat" way to increase Alexa Ranking. If you want to risk it, there is actually a URL that includes your domain name that's 301 Redirects to your URL. That's the way that Alexa counts the traffic, which is the 301 Redirect through the Alexa Toolbar.
I tested it a while back, and putting that 301 redirect URL into your signature will redirect people (and bots, etc.) through that redirect and on to your site...which then will cause Alexa to think that there is more traffic on your site.
The Alexa rankings can be "gamed" too easily. This is one reason why I don't put much faith into the Alexa rankings.
as far as i know, alexa is not very accurate, it's just a valuation, just check your google pr and pv/uv, that is more accurate options i suggest you can take more care of, synthetically think about your website, don't just do one
To increase your Alexa rank in the long run, I would highly recommended that one focus on developing quality content which attracts and maintains a large audience instead of purely focusing on artificially increasing your Alexa Rank.
20 Ways to Increase your Alexa Rank