Think about your audience: have a clear concept of who the audience is, and what is the information they will feel comfortable sharing with you.
Avoid too many fields: each field you add to the form, will cause the user to spend more time on it, increase the drop off, and thus reduce the number of submissions. Keep the number of fields to the minimum required.
Motivate: verify your users have a good enough motivation to submit the form (e.g. offer discounts, interesting articles, tips ,etc)
Clarify how often you plan contacting the users: if requesting the users contact info (e.g. email, phone number, etc) explain the users what and how often you intend to communicate with them to ease their concern from spam or frequent mails/phones
Add value with the confirmation page: after the user submits the form, use the confirmation page to offer relevant information or links to interesting resources
Choose a good theme: Select (or create) a theme that fits in with your site’s UI, but also makes the form stand out.
Get notified on submissions: configure the form so that it sends you an email notification on each submission, allowing you to followup on a timely manner