Soccer field sidelines usually have lots of trash, clad and on good days, jewelry. I start there, work my way around and eventually hit the entire field. Don't try and do it in a day because you really can't. You can hunt the same fields for months.
I always like to find out where the people sit and watch the games too. It's good to take a look when the fields are in use and make a mental note where the crowds are. They don't always use the bleachers, especially when it's hot. Look for shade trees and hills that give a better view.
Hit all around the bleachers, to the sides, in front of and behind. Same goes for the backstop area on baseball fields. Hit the warm up areas, behind and to the sides of the backstops and any grassy or shady areas where people sit on the ground and watch, and don't forget to hunt all around the benches.
I hit outside the baselines, center field, right field , left field and outfield. ANYWHERE people play is potentially a drop zone. People never seem to learn that playing sports (or swimming) with good jewelry on is never a good idea.
But seriously try and at least drive by your fields when they're in use. That will show you exactly where to begin your hunts.
Hope this helps, good luck and HH.