I'm not planning on becoming a programmer anytime soon, haha, so how do I not forget my knowledge of C++?

Just like most anything else in the world, continue to apply it. Specific to programming, continue to design applications that interest you or people are in need of.

Edit: Not really a better way, just an alternate way of saying what I said.

Even better than what Agent F said: Keep learning. I doubt anyone knows everything about even one language. Or learn other things that need your C++ programming. For example, I'm learning PHP, and while I'm doing it I'm actually learning more HTML, which I've known for 4 or 5 years. If you stop using something, you'll forget it, guaranteed. I've already learned and forgotten Liberty Basic and about a month of learning C#, as well as a lot ActionScript (I learned a lot of that in just the time the trial version thing lasted), and I'm only 16!