How do you protect your customers information

I have a small one man develop company, as time go i have more and more recurring customers where i have access to there ftp / server etc with logins, most are small customer what do not have a secure access politic and expect what i access where systems if they have a problem without asking for where logins again Right now i have a very simple way to store url, passwords etc, by writing it down in a book. But are wondering how to do it more secure but still with ease to find the information in urgent cases How du you handle it ?