You can pick up large trout in reservation stillwaters in May. Many of the rainbows will hang around rocky shallows in early May as they go through a fake spawn: they go through the body changes and even lay eggs, but the eggs will not hatch because there is not enough oxygen. To find fresher fish, move away from the shoreline.
Scuds--orange, green, pink, burnt rust, you name it--are effective, as are damselfly nymphs, beadhead Princes, Egg Sucking Leeches, Stillwater Nymphs, and most anything else properly presented.
Watch the wind; if it comes up, these lakes can turn turbid.
As warmer weather settles in, you can expect trout to move farther from shore and be active in the middle of the lakes. However, never take the weather for granted.
For more on May tactics and flies, see the Lakes in General forecast