When playing at milsim events, I strive for authenticity in my appearance---appropriate loadout for the particular scenario, accurate or plausible uniform, replica weapons, and so on, but that's not the complete picture. I think that if you are going to portray a US marine (or other character in a milsim game) you should talk and act like one, too, to properly round out the character. So here are some useful links in that regard:
Spell like a marine: Military phonetic alphabet (from 1913 to the present).
Tell time like a marine: Military time
MilTerms - an online dictionary of US military terminology, acronyms, and slang.
A directory of military glossaries for different branches and countries.
Any other suggestions for resources on character building?