Hard to guess, easy to remember is the key rule. While choosing a password one should always remember that it should not be too obvious for people who know you to guess it out. It generally said that your password should be your name, last name, nick name, username etc. Most sites now prevent you from keeping these as your passwords.
Some sites now show you how strong is your password. If it does not fit to the criteria of that site then you need to choose a stronger password. For this some site makes it mandatory to use Numeric Characters in your Password, Alphabets in capital letters, use of Special Characters like @, $, #, & etc.
After the name, people use dates as their password. It could be date of birth, anniversary date, child’s birth date etc. making it again easy to guess.
Also, what people generally keeps as their password is what they really like or are fond of. Like if someone likes some kind of food say pizza then he keeps his password like pizza, pizzabase or something related to pizza. In such cases (which are highly possible), people know that this person likes this and can guess your password by making few combinations. Why people choose their passwords in this way is because they are fond of it and they can easily remember it. To make these passwords safe as well they can do a twist in these passwords. By taking pizzabase as our password the person could have Pizzabase, PizzaBase, pizzaBase, p1zzabase, pizz@b@se etc. as password. Many combinations can be made by having capital letters, numerals, special characters. In such a way password is neither difficult to remember nor it is easy to guess.
While choosing dates as passwords, same things can be applied to. Like if the date is 02-01-2011. An easy to guess password could be 02012011 or 020111 etc. It could be modified to 020i2011, 02o20ll etc.