Leather: If the sofa isn't leather, having one leather chair can create the den feel, and if it's studded or tufted even better.
Wood paneling: Warm wood walls are iconic. It doesn't matter whether they're original Old-World panels or '70s remakes — I think they're fabulous either way. This dark moody room feels like a library but might double as the TV room.
Stuffed and cozy: In lieu of leather, a den might have casual slipcovered furniture. It can be smaller than family rooms, and the furniture can be larger and feel almost over-stuffed. That creates a wonderful sense of coziness and intimacy.
Sofa and chairs: Opt for a sofa and chairs in lieu of a sectional. A sectional is designed for television viewing, whereas a separate sofa and chairs can be easily arranged for conversation rather than TV. Mixing up the types and styles of chairs is a good way to achieve the look.
Or go for chairs and ottomans only for a den that's all about reading and reflecting.
A fireplace. A den with a fireplace makes a cozy room even cozier. There's something about having a fireplace in the room, even in summer when you're not using it.
Small rooms: Many older homes have awkward small rooms — maybe it's a small dining room or unoccupied first-floor bedroom. Transform it into a grown-up den.
Bookshelves: If you have a larger space, break up the room into two spaces: one area for bookshelves, a game table and a reading nook, and the other for the den.