As we known, our eyes have rods photoreceptor and cones photoreceptor. The rods are used for peripheral vision while the cones are used for foveal or central vision. Therefore, to improve your peripheral vision, you need to block off your foveal or central so that you can force yourself to depend on your peripheral vision. You can make use of a straw and two toothpicks to practice. Draw a circle around the straw with a black line first, and then stand one to two feet in front of the straw, and try to place the toothpicks into the ends of the straw with your focus fixed on the black line. Make sure that you use your side vision to notice the ends of the straw. Keep your vision relax in the process of doing this. If you find it difficult in the beginning, you can use a shorter straw and change a longer straw after you master it.
One of the good ways to improve your peripheral vision is to force yourself to depend on your peripheral vision by blocking your central vision. By doing this, your vision sensitivity and range will increase. You can further practice your peripheral vision by enlarging the tape in the center of the lenses. You can also ask a friend to move across your peripheral vision field. You rods will be sensitive you his/her movement. If he/she disappears, let him/her move back till you see him/her. Make he/her to stay and then move again.