Here's three tips on usability from my interview on the subject with TechJournal: http://www.techjournal.org:
First, create the simplest version of user interface required to accomplish the task you are designing. Take out everything that isn’t necessary to accomplish the task. Simplify.
Second, streamline any process toward an obvious end point for the user (enter this data and submit a form, sign up for a newsletter, like a Facebook page, buy something). Make everything modal and "chunk" steps in a process in obvious ways.
Third, don’t rely on “help” text. Customers do not read, especially when filling out web forms. Form labels are the only text you can rely on. We tried an experiment during a usability test many months ago. In bold letters on a form we tested, we added the text, ‘If you bring up that you saw this message you’ll get an extra $100 gift card.' No one did. If you need to use copy to explain anything, boil what you have to say down to 3 bullet points. That’s how people read on the web and mobile devices. Don’t be verbose – use terms people understand.
To add to those recommendations, a general best practice is to do at least one round of usability tests with any design, either formally or informally. A good usability test run well will "shake out" any usability issues in the design.