Metagaming and the character/personal knowledge split is a difficult thing indeed. Even the best of ways to keep it requires you to be brutally honest to yourself about your character. We do have a a lot of help available to us though (and again I'm stating something that might be blatantly obvious); your skills (and Lore, Spellcraft and Survival in particular). You don't even have to make a lot of rolls.
Why? This is where it comes to the "brutal honesty" part.
Even with an obscene, for example, Lore skill, that doesn't mean that a character is fully aware of the workings of each and every thing that goes on in the Realms - indeed, the Realms are probably pretty close to our modern society in terms of just *how much information* there is out there. No one *can* know it all.
Conversely, even a low-or-untrained Lore character will most likely know how things he is familiar with works.
Again with the painfully obvious tangent, but my point is this:
The skills are an excellent way to determine a character's knowledge of obscure things; that's where the rolls come in. But they must also be compounded by a character's familiarity with subjects.
A quick and dirty way to try to determine if a character knows of something that *isn't* extremely obscure is this;
Find the proper skill. There's a lot of skills that could possibly be extended to cover at least passing familiarity with something. This isn't the part to be stingy. Go ahead. Justify it to yourself. Take your skill bonus and add 10 (those familiar with PnP will know this as "taking 10" on the roll). That's your "skill knowledge" number. This number you'll then compare to the numbers here (they're taken from the Players' Handbook, so I'm not just making them up
).
Really easy: 10
Basic: 15
Tough: 20
Obscure: 30+
Keep in mind also that if your character has no ranks in the skill, it's often unlikely he'd know something beyond the "really easy" (10).
Now is where you have to be brutally honest. Ask yourself "How can my character know this?". This goes doubly if you're up poking at really difficult things. Maybe the skill knowledge potentially indicates that your character knows how something works in a specific small village over in Kara-Tur or Maztica or in a deep and far away corner of the Underdark. But how could your character have come across that knowledge? Did he travel there? Did he read it in passing in some strange book?
One really has to swallow one's pride here and admit to oneself that your character might not know everything.
Also it's worth to remember that a character might at high levels know almost everything there is to know in a specific field, but as tempting as it might be to think of it that way, Lore does not make you omniscient. Lore is probably best interpreted as the "everything else" skill, for things that you can't easily place within others, or possibly rolled as backup if you are untrained in a primary skill relating to another matter. Of course you should be allowed to flaunt your extremely high skill and the knowledge it brings; just make sure you can explain it. Slaughtering a particular monster all day will not teach you the population numbers of Mulhorand...
When it comes to magical matters, for instance, you roll/check Spellcraft, if you have even a single rank in it. That's your knowledge of magic. It might be counter-intuitive at first to have one skill limit another this way, but it's important to do so in order to not make one skill the "Ultimate Skill for Knowing All", as well as to ensure all skills have their place.
However, sometimes knowledge has to be rather common, even if it's related to a specific thing. Even a farmer could know that trolls are weak to fire (but probably not know of acid).
Why? Otherwise trolls would be eating villages at will and be a very common and nigh-invulnerable pest. Imagine if cockroaches were 12 feet tall... People would know and fear that danger and rumor of their weakness (if any) would spread, and you're back to where I started; farmers knowing of fire as "the way".
In the end, you do of course have the full right to decide what your character does and doesn't know (subject to overriding by DMs, of course
). If your character is told face-to-face by another about some obscure thing, of course your character then knows that from that point and forever on, no matter what your skills might have to say about it.
Not that it's be a bad idea to put a rank or two in even a cross-class skill if you wish to represent the fact that your character might have learned quite a lot about something in the presence of their best friend...
Now, with that rambling and incoherent wall of text done, I shall go actually play instead. ;D
). 
