Security expert Alan Woodward offers this advice:
Don't choose one obviously associated with you
Hackers can find out a lot about you from social media so if they are targeting you specifically and you choose, say, your pet's name you're in trouble.
Choose words that don't appear in a dictionary
Hackers can precalculate the encrypted forms of whole dictionaries and easily reverse engineer your password.
Use a mixture of unusual characters
You can use a word or phrase that you can easily remember but where characters are substituted, eg, Myd0gha2B1g3ars!
Have different passwords for different sites and systems
If hackers compromise one system you do not want them having the key to unlock all your other accounts.
Keep them safely
With multiple passwords it is tempting to write them down and carry them around with you. Better to use some form of secure password vault on your phone.