Girlfriends are a strange animal...depends how well she's integrated herself with the other spouses in your unit. Would introduce her to some of the other significant others before departing...add her to the FRG call list (if she likes). That way she'll get the calls etc on when you're coming home and any unit social events or get togethers they may be having while you're gone.
Def give her a mailing address. Depending on where you're going and what you're doing, you may or may not have e-mail access...which will help help or hurt.
Would def plan on writing her while gone.
Also, Saw a book this weekend,
Separated by Duty.
Haven't read it, but it looked decent. Check out the
site.