A mistake people make when changing their password is just changing one element or adding one character to their current password. Apparently, hackers program for that.
A better option is to have your password made up of four unrelated words, like this:
You can remember it by creating an image: picture a bunny going to a gym and it's blurry but what suddenly comes into focus is an elevator.
Of course, if you have lots of sites with passwords, remembering different combinations for each one gets harder.
Instead of writing down the actual passwords, you could do it with doodles like this:
However, if you look at your pictorial clues after not used it for a while you might think the password is rabbitbarbellmagnifyinglift.
Or you can use password software. I'm using Dashlane and so far I'm happy with it.