In the backyard, I have a tire buried halfway in the ground. Basically, I go out aiming to hit the tire a certain number of times. This week I'm at 275 strikes. I time how long it takes me to reach that number, then the next session I try to beat or at least meet that time.
I get really focused while I'm at it, and I try to hit the tire with near maximal force every time. When I'm out of breath, I rest a bit, then start up again.
I might do 35 strikes, then 35 more, then 30, then 35, 15, etc. Oce I've hit the damn thing 275 times, I'm done.