"Match the hatch"
or failing that cut through the mustard with something completely different.
A poor imitation usually won't work but good one is deadly. Often a black marabou wooly bugger will get them to eat better than something close, but not close enough to what they're rising on.
More important than fly is presentation, make sure the morsel comes to them as they want it. Watch them for a while, see how they rise or hunt subsurface.
It's a game that will provide a lifetime of challenges and rewards.
IFFF Certified THCI @ 2005
Capeflyfisher Guide Service
Island Hopper, Guitarist, Incurable Dreamer
and Founder, Worldwide Flyfishing Forum