Remember these 10 usability principles when creating/reviewing key areas of the user interface:
1. If in doubt, leave it out
2. Shield users from the complexities of the system
The user interface should provide a level of abstraction away from the inner workings of the system, performing a translation between what users want to achieve and the technical functions that make it happen.
3. Speak the users' language
The user interface is fundamentally about communication. You have to communicate clearly, using terminology the user will understand and in a way that allows them to take the necessary action and continue with what they were doing. This effects navigation, notifications and messages, button text, form labels. User research can help uncover the language that should be used.
4. Know your real users
Find out who it is you are creating the website for. Who are the end-users? Do some research, find out what, how, when and why these users are using the system. Talk to them, watch them, crunch some server logs, do a task analysis.
5. Do a few things really well
Getting to know your users should include some kind of task analysis, find out what the most important tasks are that your users perform. Prioritise and focus on these key tasks, do them well. Customise the interface, automate some tasks to simplify processes, whatever it takes but make doing those tasks as easy as possible for users. Allow them to initiate those tasks from the home page.
6. Be consistent
The user interface should be consistent across screens, pages and components. This includes navigation, buttons, form controls, text styling, link styling, form layout, terminology and feedback mechanisms e.g. alert boxes.
7. Stick to established conventions
Not only should the site be consistent within itself, but it should also be consistent with what users are likely to expect from other similar sites, such as the placement of the search box etc.
8. Create an effective "home page"
A home page should contain useful items related to the user's typical workflow. Use this page to give easy access to key tasks, by prioritising all the things that the site can do, and giving prominence to the core tasks users need to perform (displayed bigger, higher on the page and perhaps with icons). Infrequently used or less important items can be smaller and further down the page, if they're shown at all.
9. Get a designer in on the action
Don't leave the design of the user interface up to the developers who are building/installing the system. Even if they do have the required skills for interface design, they often lack the perspective because they have been involved in the nitty gritty of the inner workings of the site.
10. Don't release something half cooked
There is usually a really strong push to do the bare minimum and then fix it later. That is, launch the site out-of-the-box and then "take care of usability later". This is a bad idea.
Firstly, launching something that isn't working well will not ingratiate you to your users nor give them much faith in the new site. Secondly, fixing usability problems later on will require users to re-learn the system all over again. Thirdly, "later" rarely ever comes, especially if the business has just invested a lot of money in buying/building a site, they will naturally expect something that is finished and the best it can be. This doesn't rule out continual improvements to the system--including the user interface--over time but it means the site should be usable and useful from the beginning. The user experience needs to be taken into account when planning the implementation of the site and shouldn't be tacked on to the end, otherwise there will never be enough time or resources to include them before launch.
These tips are just the start, the obvious things. They can help you improve the usability of your website and concentrate on the key things that need to be done. Don't get caught up in "featuritis" or go over the top with future-proofing e.g. including requirements for things your users might use one day. Make sure your site does the key things well, where possible use scenarios to help you evaluate.