Great thread idea
I can only think of 2 right now....
1. Keep your negatives safe! I lost all my b&w negatives from my trip to Paris from water damage, so in the film days; this meant a small water/fire proof safe. Today, this means digital backups, when I first started in digital photography I was clueless when it came to backing up my data. Luckily I have most of my photos, but I have lost some good ones along the way; including the raw file of my all time favourite photo.
2. Take your time. Getting started on digital, I would snap like a madwomen. I figured it would improve my chances of getting the shot, but in reality it made me careless. When I started slowing down, thinking about my shot and taking only 1 or 2 photographs; my photography improved dramatically. As a bonus, less pictures to process