Paintballs make good practice ammo.
Use the slingshot like a bow. anchor the sling (where the ammo is held) in aproximately the same spot near your dominant eye every time you draw. Turn the fork sideways and aim between both sides of the fork. This will give you more consistent hits on target. Your sight picture should have your target between both vertical arms, only on a horizontal plane.
Other fun ammo. Ball bearings from old CV axles, lead round balls (.36 or smaller), hex nuts, etc. Use your imagination. Enjoy!