Please keep in mind this answer is not supposed to replace any of the other or to be a complete answer to your question, but I'll try to outline my approach to handling the similar issue.
I've been Storyrelling (GMing) a few similar campaigns in World of Darkness system, which is I believe perfect for this kind of premise. The idea of each one of them was that players create a bunch of mundane people, living mundane lives and as the story progresses, they discover the supernatural part of the world, which has been hidden to them.
Little by little they find themselves more and more involved with "the other world" but the idea is that at least for some time they will struggle to walk the line between supernatural and mundane. To what extent - it's up to them.
Step 1: Create a rich mundane background
Supernatural should NOT be something that will change their boring life to an interesting one. The players need to have a rich backgrounds, families and friends, jobs, responsibilities and roles they play in the society, people they depend on and people that depend on them. Having a bunch of hobo orphan PCs that are doing basically nothing until that hooded man approaches them is a recipe for disaster.
"Sacrifice" the best part of the first session for mundane life development. Let them meet in a mundane situation, present a problem to them (even a simple one, like one of the PCs house burns down, or if they work at the same company, it's about to go under). However, happiness should be abundant in this world, and the aim is to make the PC want to carry on with their lives. Do not describe Buffy washing dishes, instead, let them be a valuable asset to the sports team or a successful rockman.
The problem is that the PCs might not be interested in these kind of activities just for the sake of it. Provide rewards, resources, monetary gain, retainers in the form of interesting, specialised NPCs, like "the outdoorsman" friend, the gun nut, hobby botanist or med student. Make sure that those assets are vital to their future supernatural quests and that they complement areas where PCs are lacking.
Step 2: Foreshadow
Impress how important this mundane life is for them. If you introduced them to some supernatural stuff, impress on them how disruptive this is for their life and how easily dabbling with supernatural can get them from safe, stable life to rock bottom.
Let them meet an NPC who had his life destroyed by supernatural. Be it a broken vampire hunter, who is on a verge of committing suicide before they finally get him, as they did his family and friends. Show them a mumbling deranged hobo, who speaks about "the setup" and "reptilian establishment", when you know that indeed an alien replaced the town's mayor.
Let the players see how they will end up if they commit too much to supernatural affairs.
Step 3: Introduce the supernatural
No one goes from being a high school teen to being a full blown werewolf hunter overnight. Show the players a bit of supernatural, let them witness a vampire attack or some ghost manifesting, but do not let them interact with it too much. They have to know something is out there, but make it bigger than they can handle. Little by little, they will start seeing more and more areas where supernatural influences the mundane.
Make it relevant to their mundane life, but with small stakes - maybe there is a witch who wants to use a bit of arcane power to curse their football team - and get the sweepstake. They will probably act to "save the day", if not, let it be, there will be more of that to come. Describe the outcomes and make them aware that supernatural is successful and covert.
Also, make sure they will need to remain quiet about it and that revealing the monster would actually make things worse. If they try to do it anyway, they should be ignored and ridiculed by the mundane people.
Step 4: The slider
If you don't want the teens to stockpile on stakes and silver bullets from the first glimpse of a fang, let them feel that if they fall into this world, there might be no turning back. It could be done by an advice from a wise uncle who chose to ignore the witches and weres, whatever you like.
Establish some sort of a slider in your head, with "MUNDANE" on one end and "SUPERNATURAL" on the other. Players actions will be to remain in the middle. If they are pursuing it unilaterally, this aspect of their life should benefit but at the expense of the other one. Should they go to the extreme in their commitment, let them see the other end crumble.
They undertake to clear the city from vampires? They succeed, but their families struggle and they are never present to help, causing some regret. They try to work out digital-mystics' conspiracy in the authorities? Techno-freaks target their friends credit score, now his house mortgage is defaulting to the bank, as a revenge/deterrent.
On the other hand, they choose to pursue mundane affairs, the digital mystics get the local government under control, with new harsh policies in place affecting everyone. Or a marauding group of werewolves establishes a den in the forest, threatening the community and by extension their mundane life.
Make it a choice between the ones close to them or things only they can handle and make sure they do not have enough resources to handle both. Think Spiderman, is Aunt May more important than Venom's activities? Well, sometimes she is. Note, that they should get much more rewards and positive outcomes from their efforts in mundane life. Make it so that they gain resources from mundane life and spend them on supernatural one.
Step 5: Pull and Push
Both of those lives should pull them closer and push them away at the same time.
Mundane life should pull them with safety and success, happiness and resourcefulness. This life has a better reward to effort ratio and is mostly about positive reinforcement.
However, it should also be a bit disappointing, with a sense of lone-hero alienation, as no one in the mundane world can truly understand them. The double life is a burden here, and mom calling to see why you're not home is a nuisance when you are stalking a werewolf.
Supernatural life should pull them with mystery and power (that comes at a price), enticing characters and drama. However, this life is also dangerous and taxing and the effort spent there rarely pays off.
The idea is that whenever they make a decision, both worlds should oppose that. Doesn't matter which they choose. From that point there will be two interesting resolutions - either they commit fully to one of the worlds and the other one becomes a haunting past that pesters them all the time or they straddle that thin lane, trying to juggle too many balls at the same time.