Invite friends from home who you know really well for a meal to to see your room - no tip really expected but really really appreciated.
Invite someone else's friends or people from back home you don't know that well in for a meal or to see your room - tip expected.
Invite girlfriend/potential girlfriends family or friends in for a meal or to see room - no tip expected but really appreciated.
Send out yearbooks - letter with thank you and money expected.
That was my experience.