Accessible methods for showing help on forms

Hi all

I was wondering if I could open up a discussion about the most effective way to provide accessible context sensitive help on forms, at the input level. For example, an input for a form requests information to be submitted in a particular form, or there are rules around the information to be provided.

The input may have an associated help icon that the user can click or hover over to see the associated help for that input appear.

So, my question is - what is the most effective way to do this, and what are the advantages and disadvantages to the various methods?

Examples I can think of are popup windows with explicit close, JavaScript enabled hover over, title text, and so on.

Please let me know your thoughts on the most effective way to achieve this type of context sensitive help.

Thanks all and look forward to hearing from you.