If you're comfy with knowledge of what is in your pond (such as structure, cover, depths, water quality, and species of fish and average sizes and size ranges) then forage, forage, and forage.
Build some structure to concentrate fish around, add some hiding places for the new fish, and add forage, forage, and more forage
You're North, consider trout fingerlings as forage in addition to the bluegills and yellow perch.
