Use your hand as a ruler.
Measure your hand from your fingertips to palm and memorize it. Now you can judge the size of anything without a ruler. Try to pick a finger that's pretty close to standard length (~1 inch).
More General Tips
Narrate tasks out loud if you have trouble remembering whether or not you completed them.
If you keep forgetting that you closed the garage door, when it closes, say out loud: "The garage door is closed". Or when you wonder whether or not you unplugged the iron. "The iron is unplugged". Your brain will become more stimulated to help build a stronger memory.
Good work ethic requries good rest ethic.
"Work hard, play hard" is probably the most mainstream version of this ideology, but it's absolutely true, in order to be a top performer at anything. The most productive people regularly take a step away to process what you are doing and plan out where you are going. It also helps avoid burnout.
Want to stop walking into people on the sidewalk and in crowds? Avoid eye-contact.
Staring directly at your destination instead of other people in the eye will signal to them to get out of the way.
If you have to remember to bring something with you, place it next to your shoes.
You'll never forget it. Place it inside your shoe if necessary.
Wishing things were different is a great way to torture yourself.
No time is better than the present to make changes. Stop thinking in the past and continually look forward to the future.
Don't cheap out on what you put between yourself and the ground.
Shoes, tires, beds, office chairs, couches, etc. You will thank your future self for doing so.
For places that charge a cancellation fee on short notice, try rescheduling for a later time instead.
Once you find something you've spent awhile looking for, place it in the original spot you looked for it.
The more you concentrate on positive things, the more momentum you gain of positive thought.
It starts with a thought, works into an idea, and manifests itself into a smile.
Never lend anything that you can't afford to lose.
Always consider it donated. It will set your expectations lower especially if it comes back in worse condition than when you loaned it.