A corollary to this, don't ask users for information that you don't need.

There are a number of popular web services (Spotify was the most recent) that I haven't signed up for because they keep asking for information I don't want to give them or had to think about, such as gender. Every time I'd start the sign-up process, fill in some information, then be confronted with a question where I wasn't sure what the answer was or why I needed to provide the information, and give up. This even delayed me creating an email account by about a year.