Web forms are the web equivalent of the sales agent in a store. Many anonymous people are entering the store and the sales agent wants to convert them into customers. The agent has his smile and is ready to help prospective clients with anything. Web forms need to have this same positive demeanor, to be helpful and inspire confidence.
Hanging lots of shiny logos from MasterCard, Visa, VerySign, SSLsecured, iTrust, etc. on your site can be helpful, but it’s surely not enough to gain user’s trust. Also, putting too many logos on your form will confuse users. They will not know with whom they are dealing: Visa, Paypal, VerySign, iTrust, CCBill, Oracle, Google… Limit to only what is necessary: payment methods logos integrated into the form as selectable options and one third-party verifier (VerySign, iTrust, etc). The logo of the trusted authority should have a link that will redirect users to a page on the authority website certifying your SSL certificate, your URL, and your form. Also, asking for things that are not necessary or are too personal will, most likely, drive users away.
Don’t lie. Offering a “one month free trial with no contract and no obligations” but then asking for credit card details on the signup page is a lie. It will bring more clicks on the signup button, but the conversion rate will be small. Better to be honest and gain trust by not asking for credit card details right from the beginning. Those can be asked for later, saying something like, “Your free trial will end in two weeks. To ensure continuity of the service, please provide your payment details. You will not be billed before the free month ends and you can still cancel the payment at any time.” Offer cancel buttons in visible and intuitive locations. If you don’t lie, and don’t try to trap users into shady subscription plans, and you will get trust. Users are more likely to spend money and recommend sites that they trust.
Don’t be cryptic. Do not expect users to know what exactly you had in mind when created each text field from the form. Clearly label each form field. And most of the time, just a label is not enough since you have only about 10 words to work with, so you should create tooltips. Simple question-mark icons that display tooltips on mouseover and explain what should be filled in each field and why should be filled. Even if most of the users know, or at least have a clue about what they should put in that field, having a complete explanation will make them much more confident. Explain everything; babysit your users and it will pay off.
Do not put labels inside text fields. Use tooltips to explain what the field should contain and use separate lables for the name of the field. Caroline Jarrett wrote and article for UX Matters that gives a deeper examination of the pitfalls of putting labels inside text fields.
Make it pretty. Some people disagree with this, saying that it is not worth it to style a page that will only be seen by user once. Others argue that users have already made their decisions prior to encountering the signup form, and so their choice of whether to use the website or service based isn’t affected by how nice looking the signup form is. Those are both valid points, but the main goal of a signup form is to gain trust. Having visually appealing forms will show that the experience was well and intentionally designed, and is not just a random collection of input fields put there to trap customers.
Like a real sales agent, your web forms should inspire trust and be helpful. The effort involved in transforming an untrusted and unhelpful form into a trusted and helpful one is relatively small and it pays off very well.