Search Engines work in many ways. They first need to be able to read your code, and sit content. This is normally where 70% of sites fall down as they're incorrectly coded and make it harder for directories to search and index their content.
Then there are SEO techniques to help your rankings with particular search terms. Keywords play an important role here BUT there is a limit on how many you can use (this is where SEO 'experts' come in and exploit clients)
Then there is the hits, traffic and links. The more hits, traffic and links to your site you have, the better off your ranking will be.
Directory ranking and search optimisation done properly is a long and tedious task. Those claiming to make it quick for you are usually in for a quick buck and I really beleive you need to look at the fine print before signing any contracts.
Don't be fooled by 'statistics' etc that you're getting (or should be getting) for free or minimal costs. I have seen it a hundred times and wish with all my heart these 'vultures' would go away, but alas, if there is a quick buck to be made, you can rest assured there will be con artists in their droves.