The most basic thing you can do is to create a piece of code on the landing page (where you want customers to go), that recognizes which one of your banners, buttons or content pages generated the visit. In other words knowing which e-marketing piece is having more positive effects. With this tool, you can, in time with statistical data, start cutting the e-marketing pieces that are not working and moving this money to the ones that work. Most e-marketing providers for content e-marketing or sponsor links in search engines allow you to put this piece of code in your landing page, so you will know which investments are paying off. Now, this will depend on how you are paying for your e-marketing (per click, per conversion, per lead, per impression), but the idea is to know how much are you selling for an specific investment. This is the concept of ROI (or return on investment). You can also know, how much cost you to get each customer. So if you pay a $100 for a XX impressions and they generate 2 customers, you know that each customer cost you $50. If you cross this data with your average sale in dollars and your markup per sale is, say $40, then you are losing $10 per transaction. Hope this info helps!

Google analytics is a free to use tacking code which you place at the bottom of your html code. there is also a funnel system which you can set up which tracks people as they go through the order process and if the complete the order or not, and if not where they go. i would recommend google analytics. if you want some advice on seo check out my site at, http://www.999affiliates.com/seo