"The player whose sole objective is to contest a mark shall be permitted to do so"
Free Kicks in Marking Contests (Laws Applicable)
15.4.5 (d) A player makes prohibited contact with an opposition player if he unduly pushes, bumps, blocks or holds an opposition player or deliberately interferes with the arms of an opposition player, who is in the act of marking or attempting to mark the football.
15.4.3 (e) A player may make contact with another player if such contact is incidental to a marking contest and the player is legitimately marking or attempting to mark the football.
Your focus needs to be on the marking contest ensuring that you are observing where the players are coming from and where the contest is going to be held. It is important to be on the look out for players leading for the football and work to a position 20-25m away and side on to the contest. By making this position you will give yourself the maximum opportunity to adjudicate the contest correctly.