Make a List
How many times have you gone on a hike, hunt or fishing trip only to find out you have left something at home? It's almost inevitable that something will get left behind. You can stand around just before leaving and rack your brain trying to think of everything, but when you get into the field, you realize you left something behind.
What I have done is to create a list for all my outings. Whenever I get ready to go on a hike or hunt, I print out a copy of my list and as I get packed and ready, I cross off things on my list. If there's something I don't need (like ammunition on a fishing trip) I just cross it off up front.
I print myself off a copy of my list and the first thing I do is go over the list and cross off anything I know I won't need. That leaves only what I want to take. Then as I start packing, I cross off each item I load. When I'm ready to go, my list should be completely crossed off. I usually come home after an outing and find my list laying on the counter somewhere, completely marked off.
As I find something new or buy something, I type it onto my list so I will always have it. As I get older, the more valuable my "list" has become. It's easier all the time to leave something important behind.