Skip the optional fields and think of other ways to collect data. Always ask yourself if the question can be inferred, postponed, or completely excluded.
Data entry is increasingly automated. For example, mobile and portable devices collect large amounts of data without the user being aware of it. Think of ways to take advantage of the social and conversational user interface, SMS, email, voice, OCR, location, fingerprints, biometrics, etc.
Make it fun
Life is short. Nobody wants to fill out a form. Be talkative. Be funny. Little by little. Do the unexpected. The role of the designer is to express the brand of his company to provoke an emotional reaction. If done correctly, the completion rate will increase. Just make sure you don’t violate the rules mentioned above.