If it is not possible to avoid [captcha](http://en.wikipedia.org/wiki/Captcha) entirely, please at least do it right!
* Do not mix letters and numbers. It is hard to distinguish O and 0, 1 and l or in some cases even G and 6 or 9 and g. Please use either letters or numbers exclusively.
* Do not mix uppercase and lowercase letters, or don’t be sensitive to case. It is hard to distinguish O and o, C and c, s and s, etc. Especially when the letters are twisted, then it is no longer possible to depict case from the letter height alone.
* Do not use random words, do use common dictionary words. It is a lot easier the user. It is not that people will brute force your captcha. If some people do, simply give them a new captcha for them to break.
Thanks for your cooperation :).