Sitestone

Jared Spool has some great advice on account sign-in: 8 Design Mistakes to Avoid

1. Having a Sign-in In The First Place
2. Requiring Sign-in Too Soon
3. Not Stating the Benefits to Registering
4. Hiding the Sign-In Button
5. Not Making “Create New Account” or “Forgot Your Password” a Button or Link
6. Not Providing Sign-in Opportunities at Key Locations
7. Asking for Too Much Information When Registering
8. Not Telling Users How You’ll Use Their Information

As simple as a sign in form might seem, each small mistake can make the experience a lot worse for the user. And in some cases that can mean loosing a potential customer. In the comments on his weblog there are some more good tips.