Since everybody can configure his/hers own captcha's with a few words:
1. Use as many fonts as possible.
2. Keep the background and the font color sort of the same (minimize
contrast).
3. Use a large dictionary or random characters
4. Use a large captcha. Smaller ones are more easily broken.
5. Make sure the fonts overlap or are intersected with (by?) noise.
On the other hand if you are not running yahoo, googlemail or similar
system chances that someone is running an ai program against you are slim
at best.
