If they are too easy, you need a slant-board. Your feet are hooked under a padded bar at the top and elevated 1-4 feet while your head is at the bottom of the slant-board. I used one at the gym back in my marathoning days. Now the regular kind are difficult enough for me. If you don't have access to a slant-board, you could try holding a weight such as a medicine ball above your chest as you do them. That will also increase the resistance and is easier to do at home.