Making HTML forms easier for the user is no easy task.
Make good use of Radio buttons, Check boxes and drop-down menus within the form instead of text boxes where possible to keep the users typing down to a minimum.
Use the "Tabindex" HTML form attribute to set the tabbing order if your HTML forms are large and complex.
Make clear which elements of a form are compulsory with and * or similar, stating what the asterix is for at the top. If you think any of the fields might be filled in wrongly, then think about adding help pages (pop-ups) for the form.
If you already have data on the user then pre-populate the fields and ask for confirmation where necessary, this will save the user re-typing them.
Finally you will be able to judge your forms usability by how many badly filled forms you receive.