Actually, noplace2 is exactly right, IMO.
To the OP, you can hang streamers, banners, flags or pennants from the arm to remind you.
You can place traffic cones on the ground under the arm to remind you.
You can install foam pipe insulation on the arm to minimize the impact.
The potential reminders are nearly limitless. Be creative!
By the way, I used several of the above suggestions when we were living in a fifth wheel, to prevent me from walking into the pin box or the bedroom slide out!
Yeah, I been there, did that, that's how I know noplace2 is right on!