Making your forms more accessible is only a case of using upto 4 new tags. Let's not say new tags instead we'll call them rarely used tags :)
Yes, that's all there is to it, just 4 tags.
The <fieldset> </fieldset> tags are responsible for defining the group and placing the border around it. All the form fields for the group are enclosed beteween <fieldset> </fieldset>
example :The <legend> </legend> tags must be used in association with the fieldset tag. The legend tag it displays the group text.
example :See the html source for this form example
The label tag defines a relationship between the text and form element. You must make sure the form element has an id="something" so we can attach the label to it.
id example :See the html source for this form example
This tag is only used to on long menu items and defines groups of related items. It creates a group header defined by the label attribute.
Example without optgroup :See the html source for this form example
Once again we are creating an eviromnent which makes life easier for your visitor.
As you can see its not really that difficult to repair forms. There are only a few tags. Adding them to any existing forms on your site will yield instant benefits to your site.
Just remember, you don't have to do everything at once. Every little bit you do now not only makes your site more accessible - it makes it more usable.