Give some thoughts to your cosmology before you progress further. How do the planes fit into your world?
If your characters come from a "normal" modern-day setting, then you have to figure out how the two settings fit together. If there is some kind of fantasy universe next door, then why don't more people know about this? And if more people do know about this, why haven't the two worlds adapted to each other - in other words, why isn't magic and the planes common knowledge in the "Real World", and why hasn't the technology in the planes changed to keep pace with that of the modern day?
It would probably be best to treat the Outer Planes as some kind of dream world, just like H.P. Lovecraft's Dreamlands. This would explain a lot. Powerful dreamers throughout the ages have reached this world, which gave Earth many legends of dragons, spirits, mages, and gods - but didn't affect the real world too much. And the dream nature of the Outer Planes kept modern technology at bay - high tech just doesn't work(perhaps the tech is stuck about 500 years in the past?), and if someone somehow managed to move a piece of high-tech equipment into the Outer Planes, it soon transforms into a similar, but more low-tech item (a rifle into a crossbow, for example).
Most interaction between Earth and the Outer Planes could thus be explained away as particularily vivid dreams, mystical and spiritual experiences - or drug trips. Actual physical transportation to the Outer Planes should be extremely rare, since that requires powerful magic. In the Real World, magic should be difficult to cast - perhaps spells count as four spell levels higher, but that can be offset by higher casting times, rare ritual components, favorable locations, and so on (I hearthly recommend GURPS Cabal for plenty of ideas in that vein).
Switching back and forth between Plancescape and "mundane" adventures should be possible - sometimes you are dreaming, and sometimes you aren't. Adventures in the Real World should best be played in a Lovecraftian vein - all magic should be subtle, and the PCs should face mostly mundane opponents while trying to research the truth behind reality in ancient texts. Adventures in the Outer Planes should take on a more mythical quality, and have plenty of links to real world religion and history - including notable historical figures who died but live on in their dream selves.
At the same time, they should face the danger of being "stuck" in one of the two worlds - if they use drugs to get to the Outer Planes, maybe they need larger and larger doeses until they finally "burn out" and loose their ability to dream. On the other hand, maybe the Outer Planes become such a powerful allure that one day they won't stop dreaming and let their bodies slip into a coma...
Hope that helps.