what ID & PW? if you mean in general, it's unwise to write down any sort of password since anyone could find it.
one way is that you could use some type of pattern in your passwords, just an example that isn't great would be using a password like:
bitcointalk_blabla_123
of course you need to add another element to it, otherwise if someone found out your pasword they'd be able to log in to all your accounts.
this is a good start above. but some websites store passwords in cleartext. so if you went to a phishing site of your email providor and used
myemail_blah_123
they can then get into your email. find out who your bank is with and then realise that your banks password might be
mybank_blah_123
instead have the easy reminder for your brain.. but then HASH the reminder before using on the website
so when you set your password for your email you use the hash
255a702d1e3df60f4a0db118c4ecf77f
now a phishing site wont know that your brain can easily remember how it got to that hash. nor can it work out instantly what your other passwords will be, because they are unique to each site
for instance
myemail_blah_123: 255a702d1e3df60f4a0db118c4ecf77f
mybank_blah_123 : 68a6eca313bd2d3fb147e958fcc32b07
myebay_blah_123 : f44c5b0991d8913ad2ac0f338698ae0d