STD risk can be lowered by making sure the gums, lips, mouth and throat are healthy; not allowing cum inside the mouth when performing oral sex on a man (though pre-cum, which comes out of the penis before ejaculation, carries all the potential diseases that sperm carries); and not performing oral sex on a menstruating woman.
Some sex educators recommend that a person avoid brushing or flossing their teeth for two hours after unprotected oral sex.