Fouls (II.E): It is the responsibility of all players to avoid contact in every way possible .
Avoid contact in every way reasonable possible, while still playing Ultimate. Some contact is inevitable, but players have an affirmative obligation to make reasonable efforts to avoid contact.
A player may not take a position that is unavoidable by a moving opponent when time, distance, and line of sight are considered. Non-incidental contact resulting from taking such a position is a foul on the blocking player.
If you are already in a position, you maintaining that position is not "taking a position."