BWO      Blue Winged Olive
CDC      Cul de Canard (Duck's ass feathers)
CDCC     Cul de Canard Caddis
EHC      Elk Hair Caddis
GRHE     Gold Ribbed Hare's Ear
OBA      Olive Bodied Adams
PMD      Pale Morning Dun
PTN      Pheasant Tail Nymph

Acronyms directly related to fly fishing:

FF       Fly Fishing 
LDR      Long Distance Release (unwillingly, often due to stiff rods)
TL&CE    Tight Lines & Catch-Eat (se below/everlasting tread in groups)
TL&CR    Tight Lines & Catch-Release (often use as a ff'ers "best regards")

Common expressions:

G&T      Gin & Tonic
BS       BullShit, No flyfisher ever says this on print
IMO      In My Opinion
IMHO     In My Humble Opinion