Okay, this article delves into some heavy tech content, and I’ll admit that I don’t exactly understand a good chunk of this, but here’s part of the summary :

Upper and lower case letters/numbers in a password don’t really mean that it’s harder to break.  Using things such as symbols, letters, and numbers helps.
 Some characters in high-ASCII can add to the difficulty, such as [ALT] + 1 2 5 8   (Press and hold Alt while typing the numbers) gets ?.  This symbol might be harder to break, but it makes for an extra 5 keystrokes, and a longer password.  Instead, add five numbers/letters to the password instead.

To summarize :
Use combinations of letters and numbers
Be createive : Many people use #0 instead of O.  Instead, use () for zero.
If you write down your password, leave it somewhere safe.

A good password is more than just a complex password. A good password
is one that is not easily guessed but still easy to remember. It should
be long and should consist of letters, number, and symbols, but still
easy to type quickly with few errors. It should have elements of
randomness that only a computer can provide while still having
familiarity that only a human can provide.”

Read the full article, with more detailed tehnical information about passwords, here