Diceware is a great recommendation, but you're missing one key consideration: password reuse is a larger danger to users than is having a weak password. The Apple iCloud hack is one of the few in recent memory where a password-related breach wasn't tied to password reuse. What happens most of the time is that a site is vulnerable to SQL injection gets their users table stolen, and "bad guys" use that information to try accounts on related sites. If the compromised website was using a bad (i.e. fast) password hashing algorithm, then having a good password will protect you a little, but you're playing with fire. Password cracking techniques have been advancing exponentially, as has GPU power. But if this site is using reversible encryption or storing passwords in plaintext (which still happens with alarming frequency) then all your other accounts are at risk from the one breach regardless of how great your password is. Of course, if they're using a good password algorithm like PBKDF2 or bcrypt, even a mediocre password will be relatively safe. But what are the chances that every site you've registered with is using a good password algorithm? Probably zero. How can you check the password storing technique of a site you're about to register with? You can't.
Yeah, you could make an algorithm to modify your password across sites so that you can memorize it yet it'll be different, but as "bad guys" combine information from multiple leaks, any algorithm you come up with will be vulnerable to reverse engineering. Especially if your online identity is valuable. The real solution is to use password management software like KeePass, LastPass, or 1Password. Lock your password program with your good password from Diceware, and use unique, truly random passwords for all the websites you've registered on.