If you are having problems with finding a good and strong passwords for some sites, then i recommend that you try this site. Try to put in your passwords, and it will measure how strong your passwords are based on the algorithm that they have made. They also provide some Do and Don't section where it's an advice on how to make good/bad passwords.
I tried to use long randoms alphabets only and it only produce about half of the bar, which means it's not good enough. But when i combine it with numbers, special characters, and capital number, it turns very strong. So good password doesn't mean have to be very long (it will make hard to remember), but also a good combination of alphanumeric, special characters, and combination of capitals/non capital characters.