DeutschIndependent UX designer Erik D. Kennedy recently published an article with 15 Tips for Better Signup / Login UX on Learn UI Design – a website through which he provides acclaimed courses on both UI and UX design.
The article lists general suggestions such as autofocusing on the first field (i.e. the username, or rather: email, field), immediately validating form values, or making input labels clickable, as well as more specific – but no less expedient – ones like making password requirements explicit to users choosing a password or using different terms for the “sign in” and “sign up” workflows (e.g., “log in” and “create account”, respectively) and making it easy to switch between the two.