Activeness - You are active on codex, meaning you are not just trying to get in as a spy for another site
Contributions - You are working to make Codex a better place. The best contributions are ones which you discovered/invented/whatever by yourself.
and other stuff I can't think of but you can, so in your application post what you think makes you trustworthy, other stuff is usually not read at all. We are not always correct and you may be rejected even though you can be trusted. What you can do is during that month show you are trustworthy and apply next month, you'll have a higher chance of making it in.