I mostly fish a tailwater that has many of the characteristics of a spring creek. Much of it is C&R year round, some seasonal C&R. There are wild brookies and a few wild browns in the river, but it is dominated by stocked rainbows. When they are first stocked, they are very easy. Usually a Woolly Bugger or a flashy streamer is all you need. After they have been in the river for a week or so, they begin to forage. At that time, midge larvae and pupae, scuds, and sowbugs become the main menu items until the BWO's, hendricksons, and sulphurs become active. Imitations of the current food items will outfish other flies by a wide margin. A decent imitation, with a little flash added, properly presented, will prove to be very successful.
Ad a blanket statement, if you can determine what the trout are feeding on, imitate it, and present it properly, you will generally have some very good days.