If the negativity is already present then the first thing I would do is to sweep its presence out of the room. If you have a broom this is an ideal tool to sweep the essence of the issue out of the house but in the more modern world there is no reason at all why you cannot use your hoover and visualise the negativity being sucked up out of the room and into the bag ready for destruction.
Once the house has been swept you can look to soak up any lingering remnants of the negativity. A popular way of doing this is to cut a potato in half and place in the problem room. Leave until the potato goes black and then discard.
You can improve the feeling of the atmosphere of a room by lighting frangipani incense and of course there is always good old fashioned sage to smudge and cleanse the room.