The pale morning duns and yellow sallies are on the way out. Most of August is a trico, hopper, and caddis show.
Mornings usually begin with a trico hatch of variable length. Hatching duns will merge into falling spinners, so you need to be prepared with long, thin tippets (6X or 7X) and size 20-24 black Sparkle Duns, or equivalent, to match the duns, and size 20-24 Polywing Spinners or CDC Biot Spinners for the final stage.
Caddis will end the day. Expect evening activity from size 16-18 adult caddis with tan or olive bodies, and maybe some size-18 black caddis.
Between the caddis and tricos, cast hopper patterns to the bank. RIGHT to the bank, not 12 inches out. As close as you can make it. Even closer. Bring extra flies because you're going to leave a few hoppers hanging in the vegetation. If you don't leave a few flies behind, you're going to have poor fishing.
The best hopper activity is along grassy areas. Braided areas can be good because braids mean more shoreline per river mile, and that bodes well for hopper fishing. Yellow patterns, such as Daves Hopper, work well here. Pick a size-6 fly.
Size 20 green midges are an August occurance. Sowbugs and scuds will also take trout.
The usual subsurface patterns should produce very well in August. Lightning Bugs, Princes, beadhead Pheasant Tails, and Hares Ear nymphs dead-drifted near the bottom will do the trick. A hopper/dropper rig (a small nymph such as a size 18 natural Hares Ear tied to a 24 inch dropper leader that's affixed to the hook bend of your hopper) is a good tactic in many places.
Cranefly activity can be good in August. Larva patterns such as the Muskrat or Cranefly Larva are productive when dead-drifted near the bottom. Local adult patterns such as the Darbee Cranefly and the Ginger Spider work well when skated across the surface. See also Tying Adult Cranefly Patterns for patterns.
For more on August tactics and flies, see the Rivers in General forecast