Jared Spool interviews forms expert Luke Wroblewski covering a load of common questions about what makes forms easier or harder to use. It's a follow up to a paid seminar, but even without seeing this, there's a lot of good advice for anyone creating web forms.
If you haven't got time to listen to a 30-something interview, there's also a transcript so you can skim through to the topics of most interest to you.
Answered! Your Top Questions on Web Form Design with Luke Wroblewski - podcast interview by Jared Spool for UIE,com
Interview transcript - Web forms with Luke Wroblewski and Jared Spool
Luke Wroblewski's website