Check and adjust your mirrors and find your blind spots, the area on each side of your vehicle where you cannot see. You may not see people or cyclists when they are in these spots. On some vehicles the blind spot is so large that a vehicle could be there and you would not see it.
Adjust your mirrors so that there are as few blind spots as possible. Blind spots in most vehicles are to the back left and back right of the vehicle. To reduce the blind spots even more, position the interior mirror so that the centre of the mirror shows the centre of the rear window. You should be able to see directly behind the car when the interior mirror is properly adjusted. Position the left outside mirror by leaning towards the window and moving the mirror so that you can just see the rear of your car. Position the right outside mirror by leaning to the centre of the vehicle and moving the mirror so that you can again just see the rear of your car. Avoid overlap in what you can see in your mirrors. Because your side mirrors show only narrow angles of view, turning your head to do shoulder checks is the only way to make sure there is nothing in your blind spots.
You should know the blind spots on your own vehicle. You can learn where and how large they are by having someone walk around your car and watching the person in the mirrors.