Stretching before and after.
I was the guy that never stretched at all, and now my ankles, hips, back, wrists and neck are all fucked up at 31 years old.
I also regret learning varial flips and 360 flips before pop shove-its. I've been skating 21 years and can't pop shove-it. Even if I try really hard, it'll end up being half a varial flip. I can do almost waist-high front shoves, but can't pop shove-it to save my life. I MIGHT get one in a game of skate if I'm going really slow, and it'll be one of those low ones that barely get off the ground and then again I might not even get it. My back foot is conditioned to go for a varial/360 flip. Kind of embarrassing.
Wish I had learned a few flatbar tricks, but rails scare me. I can do exactly four flatbar tricks : bs/fs boardslide (much more balance going backside, though), frontside noseslide and backside nose bonk. I can't do anything else, even backside feebles. I scorpion'd really hard last time I tried a few years ago.
Also kind of wished I learned to push switch, but I wanted to be like Rob Welsh and Josh Kalis and go switch mongo.