As the number of websites we engage increases, and the troublemakers become more sophisticated, creating unique yet memorable passwords for each site we go to has become very important. In the past, many websites asked you to create an identity, and a password. This allowed folks to create unique ID and password for any site they entered if they so chose. It also allowed one to have an identity across a number, but not necessarily all sites. More often today, a website will ask you for your email address as your identity, along with a password. This increases your risk, as your ID becomes common across a larger number of sites, meaning crackers only have to guess your password! This is one of the main reasons a more challenging password is necessary.
The human mind likes order and familiarity - thus when asked for a password, we often choose a simple word, sometimes substituting other ch@rac+er$ for some of the letters, if we want to make it more complex. These are easily defeated by a dictionary attack - the hacker tries a bunch of common words as your password. If they are allowed enough tries, eventually they will get in. If the same password is used across a number of your sites (banking, email, etc) you can see how easy it would be to compromise your identity. So the first step is to create a more convoluted password. For the few sites you may need to access frequently, a passphrase might be a sensible option as memorable to use if away from your usual computer. Choosing a series of unrelated terms, makes the passphrase more difficult to crack, but is still memorable the passphrase Armed888InfantOctopus is far more difficult to crack, but easy to type. However, it creates passwords longer than some sites allow. Another option is to interlace two words IAnrfmaendt interlaces the first two words of the above passphrase. This, of course is far more difficult to remember, or associate with the appropriate account.
Ultimately, the solution is to keep a log of your passwords somewhere. Some folks use a paper notebook as an aid to their memory, and while serviceable, requires you search and type in the passwords each time you use it. There is also a risk if the notebook is lost or stolen, as it in itself has no security. Mac OS X lets you keep passwords in your 'Keychain", and remembers them and fills in the information at login when you enter a site. If you have other Apple devices, this keychain can be shared across those devices. Most web browsers (Internet Explorer, FireFox, etc) have similar functionality. This works well if you have one computer, and have little need for your passwords when away from that computer.
The next, and better step is to make use of a password safe program. There are a growing number of these coming available amongst the best and most popular cross platform programs are 1Password (paid) and the open source KeePass (free). Either of these are managed with a master password, and allow you to store not only passwords, but other important information. They can be set up on multiple devices, and available where ever you go. These also can be set up to automatically create passwords for you, and insert them in the login spaces when you enter websites. I highly recommend the use of an electronic password safe, or wallet for anyone with more than a few passwords to remember.
Just for fun, do a web search on the term "Top 10 Passwords".