You need to write MANY high-quality articles with relevant links, pictures, videos, polls, etc. You need to be consistent and have proven yourself an active member of the website for a certain length of time. For example, I have 25 published hubs on my hubpages. Each hub is high quality, with all the attributes I just mentioned. However, AdSense will not accept me yet, because I've only been part of hubpages for ~3 weeks. I am going to apply again after a month (or when I hit 50 hubs).