Many Faeries like to be helpful, but never thank them aloud.  A dish of milk or other offering is welcomed by some, but for others any expression of gratitude is too much. Some are offended by it and others get just a wee bit distracted...  There is a tale about a Faerie who helped a farmer tend to his fields.  Upon finding a little sewn jacket that was given to him in gratitude, he got so excited that he stopped working altogether, ran off with it and never returned.

When approaching a Faerie gathering, sit quietly at a distance until you are asked to join.  Faeries highly dislike intruders, but appreciate spectators.  It is said that one person who wandered into a Faerie market was kicked and shoved harshly until he was back on the outside, bruised and battered.

Politely decline any offer of Faerie food or drink while visiting with them or you may never be able to return home.

Never, ever lie.

Be kind, respectful, congenial and all the other practical ways in which you'd act.  Use your head and your heart, of course.

more to add...  under construction