Many many things:
Don’t pull faces, the wind will change and you’ll stay like that
Wear clean knickers, in case you get run over!
Wear a vest or you’ll catch a chill
Put your coat on or you’ll catch a cold
Close your mouth or flies will get in
Wash behind your ears, do you don’t grow potatoes (really haha)
Eat your carrots so you’ll see in the dark.
Eat up your crusts so your hair will curl
Don’t eat walking down the street, it’s common.
And my two particular favourites,
Don’t sit on radiators you’ll get piles
Don’t put cold feet up at the fire, you’ll get chilblains, I did both, I got neither!
I did wear clean knickers every day and I did get run over!