Sewing. Knowing how to mend your own clothes helps save a lot of money.
For girls, investing in a top quality hair dryer and iron. Saves you lots of money in getting your hair professionally done over the years.
Also for girls, when you buy makeup, only the part that touches your skin (base etc) needs to be top quality. Blush and eye makeup has pretty much no difference with increase in price.
And in addition, for girls, learning how to cook. Just kidding! That goes for everyone.