Karl Long has a great post at ExperienceCurve about sign-up forms.
As a marketer, my natural inclination is to get as much demographic data as possible and forms are one way of doing that. But I learned early on that less is much more where forms are concerned.
As a general rule, the less information you ask for in an online form, the more likely you’ll get someone to fill one out and submit it. If the minimum requirement of an online form is an email address, then you always have the ability to follow up with an email survey if you want to collect more demographic data on your visitors.
But the last thing you want to do is annoy your potential customers, so design your forms keeping that in mind. Always ask yourself what is the absolute minimum information you need to complete the purpose of the form. Keep it simple and easy for your visitors.