How can eCommerce (catalog) sites increase conversions?

Make purchasing as easy as possible. Amazon patented OneClick for a reason.

You can greatly increase conversions by doing two basic things.

1. Give the customer all the information they need to make a purchase in as little space as is possible. For example- when your customer comes to a product page everything that customer needs to know should be on that page. If you are selling software, for example, give the price, the operating systems, download size, etc. All that info needs to be right next to the Buy button. Your company name, address, phone number, and email contact info needs to be there to. That information inspires confidence in the cusotmer.
2. The purchase process needs to be as simple as possible. Every extra click you demand from your customer, every extra page they have to navigate to buy, decreases the number of conversions. That's the reasoning behind Amazons one click patent.

This is a two sided issue. The first issue has to do with the goods that you are selling. Specifically, are they priced correctly, can your audience find them easily, etc. This is one that you can only answer.

However the second one, is a bit more general. It has to do with your sales funnel. The key to this is to remove as many steps as possible from the time the customer clicks on the add to cart button, to the time they pay.

There was a post here (I think) with a link to best practices on eCommerce practices. I'm still looking for it, since it was one of those that opened my eyes.

1. Appearance of your website matters. Does your site look trustworthy? Have you done the certifications required for handling credit card numbers?
2. Usability of your website matters. Don't make users fill in unnecessary information. As @Hector mentioned above, minimize the number of hurdles required for purchasing.
3. Shipping charges are an important reason why shoppers abandon at the last minute - particularly women shoppers. There is survey data available to support this argument. I have blogged about this quite a bit. At the risk of being counted as self promotion, I am going to link to my blog articles which are relevant to this point. Here and here.

All of the above apply assuming you are offering something that customers want to purchase at the price you are selling them.

Keep it simple to purchase... and make some usability tests, and try to make some A/B tests for monitoring your conversion rate... I guess it will work...

Another tip is to make the website looks like safe and confortable to the users buy some stuff... some companies make the vulnerability tests for you...

This is fact, if the users trust in your website, they'll definitely buy on it.

Good luck!

Some great answers are already given, but one of my favorite tips to ecommerce conversion rates is landing page optimization. Here are some further details if you want to take a look at: http://bit.ly/6d5o1X

I'd focus on ongoing testing on every single variable you can think of, and survey your customer groups to understand their needs first.

Good luck.

I downloaded this whitepaper last year and that helped.