Note: this list is an indication of what acronyms are actually 
current on the list. they have been seen in use "in the wild". 
It is not a comprehensive list of what you _may_ use (I myself 
coined AD for dumbledore) nor a list of character initials or 
anything of the sort.

AD: Dumbledore
AK: Avada Kedavra
D-Dore: Dumbledore
DA: Dark Arts
DA: Dumbledore's Army
DADA: Defence against the Dark Arts
DD: Dumbledore
DE: Death Eater
DU: Dumbledore
ESE: Ever So Evil
GOF: Harry Potter and the Goblet of Fire
GoF: Harry Potter and the Goblet of Fire
HG: Hermione Granger
HP: Harry Potter
IMHO: In my humble opinion
IMO: In my opinion
JKR: J. K. Rowling
Lord V.: Lord Voldemort
LV: Lord Voldemort
NF: Nicholas Flamel
OOP: Harry Potter and the Order of the Phoenix
OOp: Harry Potter and the Order of the Phoenix
OoP: Order of the Phoenix
OOtP: Harry Potter and the Order of the Phoenix
OotP: Harry Potter and the Order of the Phoenix
OT: Off-topic
PoA: Harry Potter and the Prisoner of Azkaban
SS: Harry Potter and the Sorceror's Stone
SS: Salazar Slytherin
SS: Severus Snape
SS: Sorcer's Stone
V: Lord Voldemort
Vmort: Lord Voldemort
VV: Voldemort
WW: Wizarding World