100 Boxes, 50 Tries

There are 100 persons, each given a number sequentially from 1 to 100.
In a next room there are 100  boxes, numbered from 1 to 100.
Inside each box there is a note with a number from 1 to 100.

The notes are not necessarily in the corresponding boxes.

An outsider explains the strategy to the group, then enters the room, opens all the boxes, thinks for a while, then switches two notes, closes all the boxes and leaves the room.

Each person, in his turn, enters the room. He need to find his own number on one of the note. However, he may only open 50 boxes. After he found it, he needs to return the room to the way he found it and leave the room.

What is the strategy explained by the outsider?