Jakob Nielsen is an authority on usability; “the elegance and clarity with which the interaction with a computer program or a web site is designed” (Wikipedia). His tips are always worth reading and often worth implementing into your own website(s) whenever applicable.
In his most recent newsletter, Jakob Nielsen released a list of 10 redesign recommendations that are the most likely to increase return on investment. The subjects discussed are:
1) Email Newsletters, 2) Informative Product Pages, 3) High-Quality Photography, 4) Product Differentiation and Comparisons, 5) Support for Reordering, 6) Simplified Text, 7) Catering to Seniors, 
Gift-Giving Support, 9) Search, 10) User Testing, and a bonus tip on implementing a loyalty program.
Be sure to read Jakob’s excellent article on design priorities for increasing website profits.
In his summary Mr Nielsen states:
“Several usability findings lead directly to higher sales and increased customer loyalty. These design tactics should be your first priority when updating your website.”
I disagree with this statement. Firstly, these 10 points should certainly not be the first priority when updating a website. There are many more fundamental usability issues that need to be considered first, most of which Mr Nielsen has been preaching (but not practicing) for many years.
Secondly, this list should be used as a guide to desirable features of an e-commerce website, not as a usability guideline per se.