It's kind of like not letting your car get too low on gas.
Personally, I'll rarely start a Round without 30 of my chosen ball at my disposal. Obviously I'm not using them all, but it's just a habit that I've gotten into to have around 30 of my ball at a time. Once I've lost or worn out enough that the supply dips below about 24 or so, then I just "fill back up" and continue on, simple as that.
Do that, and you'll never get close to having this issue again.