To begin pick a dominating object, something like a Persian rug for example, that is a quality piece. Since the mind tends to assume then that the quality exists throughout. Then use other aspects of the room to draw attention to it, such as identical colors in curtains and chairs. Use no more than two dominant colors. In a refined house sumptuosness is in the details--details such as fringes, braiding, ropes, tassels, hardware and curtains spilling over onto the floor by 2 to 4". A professionally painted wall, called "trompe l'oeil", is a mural that tricks the eye believe there is dimension on a flat surface.
Wealthy homes often house mementos that reflect the leisure pursuits of its owners--travel, games, outdoor sports, gardening, entertaining, intellectual pursuits and collections. Collections are often displayed framed or mounted in one place. Encyclopedias lining shelves and art books are typical. Signs of entertaining would include a fully stocked dining area, a handsome serving tray or tea cart with China tea service, an espresso machine, cloth napkins and some crystal. Feeling spoiled yet?
In the backyard patio a fence or stand of bushes to form a privacy screen would be needed. Living trees in pots could be used instead (emphasis on living, not artificial). Generally a sense of quality workmanship is the goal here, not flashy or gaudy. Pretend you've never been to K-mart! Have a piece of furniture in a fine wood. In a similar vein, incorporate carved moldings in some of your rooms or a cornice and baseboard. Another architectural element that is very visible would be columns or pilasters in the living room or entry area. Plush, loose pillows on beds and couches, old sterling silver, pedestals for displaying sculptures are pretty standard.
You may need to do some homework on period furniture. I'm talking historic styles of furniture here. Choose one period of furniture that you like and copy it by buying a couple of pieces in that style. If you have a fireplace set a quality firescreen in front of it. If you have a dining room then hang a chandelier in it. They are not all large these days. Don't overlook your doors--the main entry door and interior doors. Install good hardware at least on the main entry doors. If you can get some paneled doors that is icing on the cake.
Even if you are not an art expert you can get buy a large piece of art--yes, bigger is better so long as it is not so large that it overwhelms the room it occupies. I suggest you gather opinions from other people as to the tastefulness of the artwork you choose. You will feel more confident in your choice this way. The frame around the painting should be elaborate, so as not to devalue the art. Have a small art light illuminate it. Another option for display is to set a framed piece on an easel.
Popular art subjects include botanical prints and portraits. A European tapestry adds warmth to a large room. A Chinese screen adds panache to your decor. It is actually quite useful in adding depth to a plain corner and even hides flaws in a wall surface. Here is another method of adding beauty to a wall and hiding bumps or cracks. Cover some walls with fabric rather than wallpaper. If you enjoy music then a floor harp or table harp is pretty addition. Also, an old wooden music stand with old sheet music is an option. Show an intellectual side of you with a telescope standing by a window.
In old European opulence some items don't look new. They have the look of a valued item that has been around for years and then handed down because it was special. You would notice that in some worn fabric on upholstered chairs and sofas. A tall armoire would dignify a bedroom, as would a footstool with a fur throw--faux fur would be acceptable--as it implies comfort. A full or partial canopy bed on its own would be enough to bring the bedroom to a new standard. If you want to have a mirror in the bedroom get a large on for a sense of grandeur; and make sure it has a classic feel to it.
Large porcelain animal statues and urns also add importance. A brass-studded club chair is a good addition to a man's room. A small, tasteful bar in the den is ok too. A popular standard for many years has been the double-skirted round side table. This would be appropriate for a woman's room. Don't forget that flooring shows class when it is carpeted with wool carpet or a quality synthetic blend. If you prefer wood floors stain them dark. You also can't go wrong with a real oriental rug. Favorite fabrics of the wealthy have been chintz fabrics, heavy velvet and damask. In the kitchen you can add class and utility with gourmet class small appliances, chef quality pots and pans and a good set of china, even if used rarely.
If you are unfamiliar with any or all of the terms I have used here look them up to educate yourself. Good sources of design ideas, to get you acquainted with how to put things together, would be interior design magazines showing current trends. If all else fails, hire a decorator!