Due to recent issues, the forum software now imposes tighter rules for your passwords if you choose to change them from the default one the forum issues.
Since the password isn't echoed as you type it, you may well want to type it into a text editor in case it gets rejected (because otherwise, you'll have to type it in again).
A more sensible solution is to use a password safe, like KeePass (Windows) or KeePassX (OS/X and Linux) which also generates random passwords for you.
Regrettably, the days of easy-to-remember passwords are effectively over. If you can remember it, it isn't secure. A possible exception would be a long phrase like "Correct.Horse.42.Battery.Staple". That follows the above rules (of course, don't use that exact phrase).* See Password Strength - xkcd cartoon.

* It doesn't follow the above rules any more because those words are now in the disallowed dictionary of common words. However the concept would follow the rules. Just choose different words.
Rules for passwords
- Must be at least 10 characters long. Can be up to 50 characters long.
- Must contain at least two numbers, two upper-case letters, two lower-case letters, and two punctuation characters.
- Must not be in a dictionary of the most common 100 passwords (eg. "password" or "letmein")
- May not consist of more than 4 of the same character in any position (eg. "A1A2A3A4" would not be allowed).
- May not contain sequences of 3 or more characters going up or down (eg. "abc", "456", "ZYX", "765").
- May not contain repeats of 3 or more characters in a row (eg. "aaa" or "666" would not be allowed).
- May not end with a number (so you can't just add numbers to a word, like "gorilla489")
- May not contain part of your user name (so if your name is "Barbara" the password can't be "barb9642")
Since the password isn't echoed as you type it, you may well want to type it into a text editor in case it gets rejected (because otherwise, you'll have to type it in again).
A more sensible solution is to use a password safe, like KeePass (Windows) or KeePassX (OS/X and Linux) which also generates random passwords for you.
Regrettably, the days of easy-to-remember passwords are effectively over. If you can remember it, it isn't secure. A possible exception would be a long phrase like "Correct.Horse.42.Battery.Staple". That follows the above rules (of course, don't use that exact phrase).* See Password Strength - xkcd cartoon.

* It doesn't follow the above rules any more because those words are now in the disallowed dictionary of common words. However the concept would follow the rules. Just choose different words.