It's mostly trained on the answer of other humans. The best advice is literally "just be yourself, be human."
I had some fun poisoning the well on the older text based captchas by answering the first word correctly and putting "penis" where the last word would go. It always accepted it for some reason.
The reason is that those captchas were helping to OCR-transcribe books. The first word would be known to the system and serve as the actual verification, and the second word would be unknown and serve as you performing free MTurk work.
I had some fun poisoning the well on the older text based captchas by answering the first word correctly and putting "penis" where the last word would go. It always accepted it for some reason.