If we were to measure the power of a particular medium to produce sales for you on a 1 to 10 scale, with 10 being the most powerful of all, there are only two mediums that rank at #10:
1. Word of mouth from a friend (pretty much guarantees a sale)
2. Being at the top of the search engine results (pretty much guarantees lots of visitors)
The closest competitor to these, likely topping out at a maximum of 7, would be properly targeted radio, tv or billboards, but even then it’s just stuff that points back to your website anyway (or should be).
You don’t have much control over #1 (other than being good to your clients), but you have lots of sway over #2.
CRITICAL POINT: Without a doubt the most important thing search engines are looking for from your visitors is…