I learned this trick and I would like to share this with you. If you live in the Philippines, you don't need to get lemon. You can just use calamansi.
So here are the steps to removing rust stains from clothes:
1. Mix lemon/calamansi with table salt until the consistency is pasty.
2. Spread the mixture on the rust stain and rub lightly.
3. Wait for several minutes (to several hours depending on the severity of the stain) until the stain is dissolved.
4. Wash your clothes as usual.
If the stains don't easily come off, you may have to reapply the mixture until it completely dissolves.
Hope this helps!
:)