Know your audience and target your marketing at them. More targeted your marketing/advertising efforts the higher your conversion will be. A couple other items worth noting. 1. Show the Price 2. Make it Easy to Order
There are a ton of factors that can lower your ROI. My tip would be to scratch those things that aren't working for you. For example, if you are paying for advertising on a site but aren't getting hardly any traffic from there, then I would get rid of it and capitalize on those things that are close to working (need a little tweaking) or are working for you. My second tip would be to look at where people are leaving your site. It is common for people to leave at the ordering stage, but maybe you can figure out if they are having trouble determining how to order, are you buy now buttons easily visible? Do you have a good shopping cart system? What is your bounce rate on your home page? Are they even looking at your products at all?
To increase your ROI you have to increase your conversion. As you can see thats a huge topics to cover.
Be more targeted/pinpoint/niche. ROI always sucks when you're targeting an audience that is too broad. Also, target those who are at their point of purchase, not the ones who are just doing product research.
Many people focus on how many visitors (clicks) they receive from a paid search campaign. A good indicator to know if you are succeeding, is to focus on the Conversion Rate metric as it pertains to sales generated when running paid search.
Visitors (clicks/traffic) have nothing to do with ROI (return over investment). ROI is a measure about how much money you get back for your spendings. If you spend $100 for a marketing campaign and that campaign makes you $ 200 in sales, that's a ROI of 100% which does not mean that you earned 100% because you have to take off that your product costs.
This is an impossible question to answer when we haven't got a clue what you are working on and what you are currently doing.