All of the above are brilliant suggestions. To add to them, here are a few more:
1. Negative entities are usually spirits who are stuck between worlds. Some of them have been in limbo for centuries. You need to 'send them into the light' by lighting an oil lamp or candle. Ideally the flame should burn 24/7, but if you can't manage that then light one after sunset and let it burn through the night. When you light it, say a small prayer or incantation, and ask the flame to protect you and brighten any form of darkness that surrounds you. Please let the prayer/incantation be said in your own words. And oh, don't forget to thank this element for protecting you.
2. Keep a black tourmaline in the house. Please remember to thoroughly cleanse this stone every alternate day with salt water and charge it once a week by exposing it to sunlight for a few hours. All black stones need to be cleansed and energised more than the others, because they are known to absorb negativity.
3. Whether it's your work station at office, your house or the bag/wallet that you carry, you need to de-clutter you living space and keep it as clean as possible.
4. You can also keep small cups of salt (preferably sea salt or rock salt), water and camphor in every room. Cleanse and change the water, salt and camphor, once in 24hrs
5. Before performing any ritual, whether it's burning sage, lighting a candle/oil lamp, please do not forget to have a bath.
Hope it helps