Every additional field in your form is making you losing you leads, roughly 10% per field. So consider whether each question justifies the incremental loss in leads or opt-ins. Make a clear distinction between 'nice to have' and 'need-to-have' information. For example, if you are an automotive company and your goal is to generate leads, don't ask for the current car someone owns. That's a question that can be asked by the dealership in a later phase. The goal of lead generation, for example, is to be able to contact a person.  Not to collect as much information as you can.