I just finished packing my camera bag for a shoot, and as I chose gear I thought about how packing gear was an interesting reflection of a photographer’s maturity.
Novice photographers carry everything they own. At first it’s easy, because they probably have one camera and one lens, and leaving either of those behind doesn’t make sense. Very quickly they add accessories, and those always go in the bag. And then they get a second lens, and maybe a third, and they get a bigger camera bag to carry everything. I confess that I have a behemoth camera backpack that I use fairly often.
After a while a photographer either accumulates too much equipment or comes to his senses (or both) and realizes that he doesn’t need every single piece of gear he owns with him every time he takes a picture. Often this is caused by the sheer impossibility of carrying everything he owns, but it’s still an important growth step. The photographer is now planning his shoot before he goes out, so that he can carry what he needs. OK, often it’s so that he can leave behind what he’s sure he doesn’t need, but it’s still progress.
The third stage of maturity comes when the photographer packs carefully for a shoot, uses every major piece of gear he packed, and doesn’t wish for anything that he left behind. This means that not only is he thinking about what he needs for the shoot, but he’s predicting correctly.
I’m about a 2.3 right now– I plan carefully, and sometimes get it right.
How about you?
Stop Shooting Auto! 
I strongly prefer primes, so this is a significant issue for me.
Most outdoor places I go I take a backpack with all my gear, three bottles of water, and two dry pairs of socks. Usually I carry a tripod as well. I really don’t see much benefit in not carrying two or three lenses I don’t use in that configuration; compared to the water, they’re pretty light. Sometimes I don’t use the tripod, but I end up needing it as a balance aid often enough that it’s worth carrying even when I doubt I’ll use it photographically. Plus I figure any extra weight is extra calories burned.
When I go somewhere I can dump the backpack, I take it with me, since there’s no reason not to. The tripod lives in the trunk of my car if I’m not carrying it somewhere, so it’s usually available.
If I’m going somewhere with a specific purpose, I tend not to carry a bag at all. Either I’ll glue one lens to the camera and use it all day, or I’ll put the pouch that came with the 135L on my belt, and bring one medium-sized lens plus one or both of the Nifty Fifty and Zuiko 24mm f/2.8, which are pocketable. I love the Zuiko in particular for the convenience of its size.