This guide by Justin Mifsud over at Smashing Magazine is one of the most informative and well-balanced guides to Web form design and usability that I've seen on the web. Instead of blindly proclaiming oft-heard things like 'Labels above input fields are the BEST' Justin summarizes the advantages and disadvantages of each aspect, providing useful examples and sources along the way:

Contrary to common advice, above the input field is not always the most usable location for a label. It’s ideal if you want users to fill in the form as fast as possible. But there are times when you’ll want to deliberately slow them down, so that they notice and read the labels attentively. Also, keeping a long form to a single column and making users scroll down the page is better than breaking it up into columns in an attempt to keep everything “above the fold.” Each style of alignment has its advantages and disadvantages.

Working in government, forms are a huge part of what I do. If you deal with forms, check out Justin's post and share it with your developers.

Here's a few of the books Justin recommends:

• Forms that Work: Designing Web Forms for Usability by Caroline Jarrett
• Web Form Design: Filling in the Blanks by Luke Wroblewski
• Eyetracking Web Usability by Jakob Nielsen