If possible, use a password management software. Password management software helps to use unique and complex passwords for multiple systems.
Do not reuse the same password on multiple systems. The reason is that if one system is compromised the other systems are likely not at risk.
If you must write down a password, do not write down the whole password. If you withhold two letters from the written password, it is likely the account would be locked before the someone was able to figure out that the complete password was not there. An example is below.