First he makes a conscious decision to serve everyone (v. 19).  The decision to serve is to gain at first the person's confidence (an important part of friendship), and in the end, the friend's conversion.

The apostle then elaborates further:

"To those not having the law I became like one not having the law (though I am not free from God's law but am under Christ's law), so as to win those not having the law. To the weak I became weak, to win the weak. I have become all things to all men so that by all possible means I might save some."
1 Corinthians 19:21-22

Paul's language is very powerful.  Men are without God, and without God, they are drowning.  Paul is like a lifeguard, diving into the tumultuous waters, and saving men from spiritual death.  And he does this by making friends of both Jews AND Gentiles, who are hostile to each other!  This was Paul's evangelistic strategy.

A Wife For Free In the summer of 1998 a group of Christians from Swallowfield Chapel in Kingston, Jamaica went on a trip to Tanzania.  The team of seven was actually on a missionary journey very similar to those mentioned in the book of Acts.  Their goal: the planting of a church on African soil.

During their mission, one team member befriended a Tanzanian herdsman.  It turned out that both men were interesting conversationalists and, since they were married one of their exchanges was about the question of where `Cain' got his wife:

Tanzanian pastoralist:        I paid three cows for my wife, how many did you pay for yours? (I hope I got the number correctly)

Jamaican evangelist:          Oh, my wife was free!

Tanzanian pastoralist:        Really! Can I come to Jamaica to get another wife?

And the conversation went on.  Eventually, the Jamaican evangelist led the Tanzanian herdsman to faith in Jesus Christ.

And the story doesn't end there.  Over time, a church was planted in his village.