I place out a mixture of :

1 part Di-calcium phosphate, this is a dairy feed additive bought at feed stores.
Comes in 50lb Bags at around $13.00 you need one bag.
2 parts Trace mineral salt, the red and loos kind without the medications.
Comes in 50lb Bags at around $9.00 you need two bags.
1 part Stock salt, ice cream salt.
Comes in 50lb Bags at around $5.00 you need one bag.
I also add one gallon of Lucky Buck for scent/flavor

They are killing it all summer