If you want to look after parks only dig when the ground is soft and or wet. Lets start there.
Use a philips head and once you've pp'd the target, start probing with the screwdriver, perpendicular to the ground, the sharper angle you probe the more chance you'll miss the target so probe straight down.
Cover a reasonable amount of area probing, about 2" square. How you do this is up to you, I find that a spiral pattern works best.
If this fails, pp again and try probing again.
Don't get discouraged if you can't find the target, it could be;
1. you didn't pp correct the fist time
2. a target like a coin could be on its side making it difficult to tap with a screwdriver.
3. you moved the target probing, turning a coin onto it's side. When a flat target like a coin sits sideways in the ground, it plays havoc to a detectorist's pp ability. In such a case, you'll simply have to be more careful. I like to pp by lifting the coil above the ground until the signal becomes weak and then pp.