There is a plethora of information on the web about how to design web applications but, just to get started, here are a few points to help you on the way to designing a great user experience.

• Plan before developing. Start with the user flow and understand how the application should fit together and interact.
• Focus on what you want the user to be able to do.
• Ensure buttons and menus are easy to find and use.
• Avoid clutter.
• Design for repeat use.  A web site may be visited once or twice but a web application could be used continually throughout the day.
• Provide Feedback.  Let users know when their request is being processed or has finished processing.
• Use transition to lead users into a different interface.
• Keep a consistent interface throughout the entire application.