We have so many passwords that we need to remember in our day to day life, it is very hard not to write them down. Often if you do forget your password it is relatively easy to request a new one and it can be with you within minutes by email or text, but occasionally you may find you have to wait days to receive a new one in the post.
When choosing a password try and make each one slightly different. Having the same password for multiple sites is asking for trouble, as if one account gets hacked often all of the others will too.
Keeping part of the password the same but using different start or end letters can be a great way to help you remember your password but still making it very secure.
If you do have to write your password down then be sure to encode it by adding letters / numbers around the password that you will know to ignore.