Washing clothes inside out
I bet everyone tells me they've been doing this for years, but I thought I'd check.
Anyway, the difference this makes to their condition and longevity is really dramatic. My rough estimates, partly based on items like levi jeans that I have been buying regularly for years, are indicating that it increases the useful life of clothing by around three times.
Having to buy just 1/3rd of the clothing I otherwise would is going to save a lot of money.