I can't claim credit for this information - it's something I picked up a long time ago - I don't remember where (but Kudos to the Creator :)
Next time you pull that favorite white shirt out of the washer only to find rust stains on it - don't despair - there is a cheap, easy, and effective method for removing rust stains that costs almost nothing - and you probably already have the ingredients at home.
What you will need:
Mix the salt and juice in proportions that make a kind of paste-like slurry. You want a mixture where you don't have excess juice sloshing around - enough salt to "hold" the juice, but not enough to be crumbly. There is no exact proportion here, but it's better to err on the side of too much juice since the fabric will absorb it quickly,and you want to be able to spread the slurry out before it gets too dry.
Take the paste/slurry and smear it on the rust stains - try to cover them evenly. Then, take the garment outside and set it in direct sunlight for around 30-60 minutes - and the stains should be gone. You may have to re-treat really bad stains (but I've never had to). When the stains are gone, just launder as you normally would. I've found this method works best in the Summer - the Winter sun doesn't seem strong enough.
Disclaimer: I don't know how well this would work on colored garments (i.e. non-white clothing), but I think it would be OK.