1. Content – Regular, frequent and high quality content produced by people with knowledge in the field, or people that you can trust to do their homework before jumping headfirst into writing it. This is crucial both at the search engine algo level and the PPC campaign design level.
And its not just a fresh article once a month. If you examine your log files you will see how often the SERP's comeback to visit. Once you are in good with them, they hit your site 1-3+ times a day. It will only do this if it sees fresh content, via your navigation.