Here my question is how to keep password safe so that we don't forget it.
It's no different from any other password you use online. You have to come up with your own method of creating secure passwords that will remain in your head. If your memory isn't good, then use a quality, open-source password manager.
It is true that strong passwords are important to keep your account properly secure. But as you mentioned, I don't agree that it needs to be changed often. Is changing the password the best way to keep the account safe or is it the best way to save it in the right place?
Changing passwords from time to time is a good security practice. It helps keeping you safe if there is a security incident and leak that involves an old password of yours. It's worse if your current password (that you never changed) leaks. Obviously, you shouldn't reuse the same password on other sites and services because you are increasing the risk of losing much more if just one becomes vulnerable.
Regularly changing passwords isn't mandatory. If it's a unique and strong password, then it remains like that even after 1 day/week/year. Still, the information in the paragraph above remains valid and is an extra security step.