Gina Trapani has posted a great article on LifeHacker showing how to choose strong passwords that are also easy to remember. I've been using a variation of this method for a long time and have made slight modifications based on advice I've heard from security experts.
I use 2 classes of passwords. The weakest is used for the multitude of web sites which ask you to create an account but which store no financial or personal data about you. I reserve the strong passwords for web sites which store data which could cause me financial harm if it were to be discovered.
Choose something which works for you but which offers reasonable strength. Use the Password Strength Meter at SecurityStats to test the strength of your chosen method to make sure your passwords can't be broken too easily.