The easiest way to increase your app quality (and tests coverage) is by adding a failing test for every bug before you attempt fixing it. This is a magic spice to fight regression.
It’s hard though to do it like described above. To fight the urge to dive in because you have the feeling (or you actually know) whats wrong. It’s harder to create test for a bug than to fix a bug.
But it’s not an accomplishment to fight a bug once. It’s a challenge to
– not to create new ones by doing so
– make it go away for good
Closing tip: at some point, when you’re skilled with testing, you start to notice you could explore the bug with tests… and you kill two birds with one stone (this is one awful idiom, isn’t it?).