A solution I have found helpful is to tie the keys of my car to whatever it is I need to remember (a particular bag, a camera, a shopping list) and then, since I generally will not forget my car keys, I will also remember to take along what's needed.
I have one of those clamp keychains which are easy to adhere to the handle of a bag or other object. The night before, or even a few minutes before leaving the house, I adhere the keys to whatever it is. No more unnecessary trips!
This tip is also helpful in not leaving behind said object (a camera, a purse, a briefcase, etc) whenever I arrive at my final destination. If I've gone to an event or dinner party and wish to not forget my camera when i go home, I will adhere my keys to the camera's case. Even if I go as far as leaving the camera behind, I will quickly get to my car, realize I can't go anywhere, then remember to go back for the keys - and the camera.
By ahorrasi from Marshall, NC
Related: 
Avoiding Extra Trips
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