If you leave cosmetics or a camera memory chip behind, that's no big deal. You can buy practically anything along the way, or when you reach your final destination. However, if you're travellig on a budget, having to buy a new camera or a fishing pole can be a pain.
So, a week or so before you start packing for your trip, make a list of everything that you want to bring with, and everything you need to bring with.
As you put each item in your suitcase, check it off your list.
When it comes time to leave, and you find you've got too much stuff in your suitcases, such that you decide to leave some stuff behind, check your list and make a note of what you're leaving behind. In the stress of the moment you might forget, and when you get to your final destination and can't find something you're looking for, the list will remind you that you elected to leave it behind.
If you didn't put it on the list, it's not your fault if you forget it.... so make sure you start your list well in advance, before the stress of trip planning starts to get to you, so that you don't forget to put things on the list!
Then, make a list of all your suitcases, and when each one is put in your trunk or the backseat, check it off. Missing fishing poles? Forgot the stakes for the tent? These lists will remind you of what you need to bring.