Label position: For familiar field types (first name, last name, address, etc.) It's been shown that labels placed above their fields typically outperform those that are alongside of it. If you run an eCommerce site, you need to check out this report.

Inline validation: Do users have to submit information before getting feedback? If you require a visitor to submit their phone number in a certain format, inform them as they complete the form; don't make them submit the form only to receive an error message.

Number of fields: For each field you remove you can expect a bump in your conversion rate. The downside: less information about your customers. Balance what fields you require for qualifying lead quality with your conversion rate goals.