Etymology: The simplest explanation for remembering this character is to see it as two characters - line and stop.*
Mnemonic: THE CORRECT THING IS TO STOP AT THE LINE.
* However, the original form comes from a character for lower leg 足. Your leg was correct if it was straight (!).
正解 (seikai) correct answer
正月 (shougatsu) New Year
正直(な) (shoujiki na) honest
(Based upon Henshall, 1998)