There are many sites on the web that provide help and advice in enabling the site to be as accessible as possible:

• W3C Web Accessibility Initiative
  The World Wide Web Consortium's (W3C) commitment to lead the Web to its full potential includes promoting a high degree of usability for people with disabilities.
• BOBBY by Cast
  Bobby is a free service provided by CAST to help Web page authors identify and repair significant barriers to access by individuals with disabilities. Often the many question marks appears daunting, but looking more closely you will see that it probably passes Level 1, and other problems may not invalidate your ability to display the Bobby mark of approval.
• RNIB Campaign for Good Web Design
  People with disabilities have a moral (and in some cases a legal) right to be able to use web sites. Too many organisations and designers still fail to recognise this right. There are 2 million blind and partially sighted people in the UK. Can they use your web site?
• Useable Net
  Lift Evaluation software, on which you can have a free test of 5 pages, automating the process for creating usable and accessible content as you go.