As people here have mentioned, you can tie most of King's work into the Dark Tower, and that is kind of the thematic point of the series. But really, you don't need to. I have read all of King's seventies and eighties works and very little of what came afterward but I still knew enough to "get" the references in the series.
The only thing you need to read in order is the Dark Tower series itself. Nothing in the series gives away major plot points of the other novels and it would probably be just as cool to read The Stand or Salem's Lot after finishing DT. Those of us who grew up in the seventies and eighties had to wait years for the series to finish but I think it would be a lot of fun to read all seven (or eight) books in order.
I slightly disagree, and here's why:
For me, what King was doing in interweaving much of his non-TDT writings with the 8 core TDT novels is/was exactly the same thing that J.R.R. Tolkien did with his Middle-Earth writings: he was creating a complex, over-arching mythos, of which the 8 TDT novels are both the signature 'spine' as well as, to quote the late Raynar Unwin, 'just the tip of the iceberg', which is why I want to read the 24 novels I've identified.
Having said that, though, I sat down and came to the conclusion that 7 of those 24 novels could be very easily excised while still maintaining the overall 'tapestry' of King's TDT mythos, which would leave you with the following list:
The Eyes of the Dragon
The Gunslinger (Revised)
The Drawing of the Three
The Waste Lands
The Stand (Uncut)
Wizard and Glass
The Wind Through the Keyhole
Hearts in Atlantis
Wolves of the Calla
Song of Susannah
The Dark Tower
Since I've already read The Talisman, Black, House, and the first four of the core TDT novels once, which approach do you guys think I should take in reading the remainder of the series in order to get the fullest effect, keeping in mind this idea that, while the 8 core novels do stand on their own, they're 'just the tip of the iceberg' when it comes to King's wider TDT mythology as I've already identified it?