My biggest regret would be the time I've spent in tanning beds. Thankfully, it's just been a couple of times at 2-3 months at a time, but I've definitely noticed new moles since then. I will never, EVER get in a tanning bed again. I've been a hard-core sunscreen user since I was 16. I don't want wrinkles, freckles, moles, sunspots, or skin cancer. I wear sunscreen year-round, and a higher SPF when I know I'll be outdoors for any length of time.
I also regret picking at my face when I break out. Not to say that will stop me, but I usually hate myself for doing it. I'm fair, and I scar super easily, and while I'd probably scar anyway, I'm sure picking at things makes it worse.