It's always a good idea to have a different username/password combination for every online account you create. That way, if one account becomes compromised, you don't have to worry about hackers having access to your email, your Facebook, your bank account, and the rest of your sensitive online information. One option to make sure you remember all your combos while keeping them tricky to guess is to have the same password, but different usernames for each account. That way, you can remember the password without writing it down and keep a written list (or text document) of all your various login names. No one will be able to guess the password just by glancing at the usernames, and one compromised account won't lead to a breach of the rest of them.
Alternatively, you can create a different password for each account, but base it all off the same root word. Choose one root word, memorize it, and then create passwords around it--by adding two or three numbers to the end of it, for example. That way, you can write down the added numbers on a card or document, giving yourself an easy key to all your accounts without letting anyone see your full password.
Certain websites will force you to create passwords with very strict criteria--ones that need to include a capital letter, a number, and a special symbol, for example--even though a simple series of words is often both easy for you to remember and difficult for cracking software to get past. If possible, choose a password that's a catchy phrase of four or more words in a row. If you need to write down a hint for yourself so you don't forget it, try recording synonyms for each word. Of course, you'll probably at least need to throw some numbers in there to keep security meters on websites happy, so use the above tips to make sure you can keep track of everything.