Erm, memory? You remember whether something has been introduced or not.
Take a hypothetical phone conversation, an example that nobody has managed to explain to me satisfactorily: A: "Where are you?" B: "I'm in [the/a/(nothing)] shower."
People tell me that "the" is the correct thing to use. But A is none the wiser; B could be at home, in a gym, at work... A is none the wiser about _which_ shower B is in and the shower has not been introduced previously in the conversation. It's tautological "the", B is in that shower that he's currently using, so he could just as well say "I'm in shower".
Or, similarly: A: "Where's my food?" B: "I've put it in fridge." If A is at work, he'd go looking into fridge at office. If at home, he'd go look into fridge in kitchen. If there are two fridges nearby, he'd ask "Which one?"
Even though I deliberately omitted "the/a" in the previous paragraph, I don't think you got confused about _which_ office or _which_ kitchen fridge A would go looking in. And personally I cannot imagine that A, upon hearing "it's in fridge" while being in the office, would go home and look into the fridge at home. Or that he'd even have to ask "which fridge?" if there only were one at office.
So there's an attempted answer.
EDIT: one of my rules of thumb is: if I can put "some" in front of a noun without changing the meaning, then it's most probably appropriate to use "a".