His hallmark was his "open shop" policy. He established his non-profit hospitality in the middle of the desert, welcoming every traveler and nomad with a hot meal and a place to rest.
Some considered him an extremist. Once, he was in the middle of a conversation with God Himself when some travelers appeared in the distance. He apologized and ran to invite the travelers in! For Abraham, hospitality was more important than receiving the Divine Presence.
The tradition remained in the family. When the Roman Emperor Julian ordered the establishment of hotels for travelers in every city, he referred to the example of the Jews, “among whom no foreigner is neglected.” Even in the worst of times, Jewish communities had a committee to provide food and lodging to every traveler, without discrimination.
Hospitality (Hajnasat Orchim) is primarily observed with visitors from out of town. However, it is also acceptable to extend hospitality to local guests. Since it is such an important mitzvah, one should not wait for someone to ask; one should extend an invitation.
Once inside, some guests are shy, so you should anticipate their needs and offer help before they ask.
Another key to Abraham's example: even though he had many servants, he personally attended to his guests and served them. If it's such an important mitzvah, why not take care of it yourself?
When guests leave, it's important to ensure they have food for their journey. It's a mitzvah to accompany them to the airport, bus stop, or train station, or at least to within a couple of meters of the house's entrance. In fact, the reward for accompanying guests far exceeds the reward of anything else one might have done for them. It goes beyond simply looking after them while they're in the city; it's about ensuring they arrive safely at their next destination.