Shipping costs are a barrier to online shopping, so offering free shipping can be a great way to increase sales if it doesn’t significantly erode your profits. Sometimes, sites offer “free shipping on orders over $x” to prevent significant profit erosion. You may also choose to offer this only to e-mail subscribers, best customers or promote this for a short period of time (one day, for example).

My clients have had good success with exclusive free shipping offers as well as “free shipping on orders over $x.” If you opt for “free shipping on orders over $x,” find an appropriate dollar cutoff by finding out what your site’s average order size is, then increase it by a small amount.

Customers get confused when sites don’t make it obvious that they’ve added something to their shopping cart. These days, it’s common for sites to have a “persistent shopping cart” — or a running tally of the number of items and total price of items in their cart — on every page. If your site has a persistent cart, make sure your site alerts the customer that they have added something to their shopping cart.

Many sites simply add the product with no alert, while others alert customers with messages that go unnoticed. This results in the customer asking “Did this product get added to my cart or not?” I recommend sending the customer to a shopping cart page when they click add to cart. If you choose not to do this, consider putting a bold, shaded message at the top center of the page just below your site’s main category links. Then add a link to the shopping cart just after the message.

Putting links to “similar items,” “related items,” “best sellers” or accessories on individual product pages helps provide a logical next step when a customer realizes that a product might not fit their needs.

Be careful though — this can be detrimental if these cross-sells take up more real estate and draw more attention than the actual product you’re trying to sell. Remember that product images draw the eye’s attention, so use them sparingly or use small thumbnails when advertising cross-sells.

Your nicely designed web site has pretty “add-to-cart” buttons that match the rest of your site, but are they blending in? Many of today’s graphic designers think that small add-to-cart buttons that match your site’s color scheme will be more aesthetically pleasing. But do you really want your add-to-cart button to blend in?

Imagine your add-to-cart buttons bigger and then have your designer create an add-to-cart button that is a size bigger than that. Choose a color for the button that stands out visually.

How do you know when your button is big enough? The first thing a customer should notice on an individual product page is the product image, the next is the add-to-cart button.

It’s a fact that even the “big boys” of internet retail leave money on the table, and GrokDotCom does a good job pointing out real examples of how large retailers could improve their conversion rate. My favorite is their latest screencast that shows how one of Petco’s Google Adwords ads clicks to a page that shows no products.

No matter the size of your ecommerce site, you should check out GrokDotCom’s conversion rate screencasts and see examples of how ecommerce sites fail at converting visitors to paying customers. By viewing these, you may get ideas for improving your own site.

Looking for a good read on marketing? Ad Age has just the thing for you — its Power 150 media and marketing blogs. Included in the list are some of the more popular blogs (and personal favorites) like Duct Tape Marketing, the SEOmoz blog and GrokDotCom. Our friends at RKG even cracked the list with their blog.

Here are some others that I follow, and while these aren’t on the list, they should be because of the quality of the information they provide:

• Campaign Monitor Blog: This marketing blog primarily features e-mail marketing tips.
• Get Elastic Ecommerce Blog: Ecommerce blog covering usability, design and marketing.
• MarketingVOX: How did they miss this one? Features the latest online marketing news.
• Marketing Experiments blog: A more quantitative look at online marketing.
• Smiley Cat: Christian Watson’s Web Design Blog: Discusses topics in web design.