A towel is great for this exercise. So lay out a towel at the distance your practicing (say 10-15 yards) and set yourself the challenge of hitting a number of times for example 30. The idea is, that until you’ve hit it 30 times, you can’t leave!
This exercise will not only focus your efforts, it will add an element of frustration to your practice. Frustration is actually a key element in practice as it shows commitment and determination to improve. A top sports physiologist I’ve listened to Dave Alred calls this frustration the ‘ugly zone’. So if you do find yourself frustrated and struggling to achieve the number of towel hits you’ve set yourself, commit to the task and push through it and you’ll reap the rewards.