Understand that every little delay you introduce to your customers creates abandonments. Studies have been done on page load times, additional fields, text entry errors, etc, and they all lead to the same conclusion - every bump in the road creates abandonment.
- Make your text fields forgiving. Google is a master at this. Doesn't matter whether I enter Mar 20, 3/20, 20 Mar, etc. Google figures it out. I don't get an error message saying "please enter in the form mm/dd/yyyy". Same goes for phone numbers. This is trivial to do in code, absolutely no reason it shouldn't be done.
- Make it clear why required fields are required. Some are obvious to the user, most are not. A simple "why?" link giving the user the ability to learn more about why you required their birthday, SSN, and underwear size is easy to do.
- Your user is not going read everything you show them. You have three seconds (used to be five) to give them a reason to read for another three seconds. And so on. You screw up those three seconds? Good bye.
- Your competitors are a click away. Take a look at their sites. What do they do to make things easier. Amazon allows me to return items doing no more than sitting at a computer and dropping the item on the front porch. Why do Best Buy and Walmart make me go to their store? Yes, this has everything to do with UX - your processes are exposed through UX.
Yes, there are only four there. That's the point - do more with less. Always look to what can be removed from your site to add clarity, not what can be added.