Act now while it's fresh ____ your mind.
whoeva put on is dumb
"On your mind" means your thought is kind of suspended in your mind and can be gone or forgotten whereas "in your mind" means the thought is implanted in your head and cannot easily go away.
So, "on your mind" is correct.
If your on your mind is correct, then why did some many people pick IN your mind? Explain that, gardemarine!