The first, best thing you can do to measure ROI is to measure EVERY bit of customer activity on your website. What kind of traffic are you generating? Which products are generating the most interest? What kinds of questions are your customers asking? How long are they staying on your site? Are they managing to navigate through the site easily? Are they finding the information that they came in search of?
If you're not asking these questions, you'll need to find a way to start doing so soon to better measure marketing ROI. This means measuring your site traffic and tracking conversions vs. click-through rates for any advertising you do, just for starters. Your site has to be up to date right now and updated very regularly in the future. There are new tools available to businesses and marketers to help accomplish such analyses, but, again, you have to be able to dedicate the money or manpower to doing so.
