What do I do to make my house more cozy?

Fill it with kittens.
Seriously.

For coziness, you can add soft yellow lighting and throw blankets. Those things can make a space seem more nest-like.

Warm it up with the addition of one or two bright complementary colors. Red plays nicely with khaki. What color is the furniture?

Introduce the colors (only a couple) with throws (as @Likeradar suggested), pillows on the seating, accessories such as candles, posible a rug and in the artwork.

Do you have the furniture arranged in a cozy conversational area?

Does your living room feel too sterile? Maybe you should look at the way each peice of furniture is spaced apart from eachother. Bringing the chairs, couch, coffee table, and home entertainment set close together draws the room together and makes it feel homier.

Flowering plants in pretty containers; you need sun and the memory to water them; photos of family and loved ones; some special art work, pottery, treasures from granny..

cushions…big fluffy ones :)

Buy some of those electric fake fireplaces.

scent! fragrance! get candles/air fresheners/etc…

Feng Shui is kind of complicated, but some simple rules make since. Color can really effect your mood. I also have noticed that flow really does make a difference. They talk in Feng Shui about the direction of the energy a lot. Is there a direct line from the front door to the back patio? Feng Shui says that the energy is going right out the back door. Break it up by putting a sofa or something to “capture” the energy and direct it back into the room. I don’t understand it, but it does seem to make a room feel more comfortable. It could be interesting to do a little Feng Shui googling to see if you can figure out what it it about your room that is effecting your mood.

Here are some ideas about color from this website. Color can really effect your mood.

Red: Great for energy and excitement. Works perfectly in a dining room to keep folks awake at the dinner table. Not so good for bedrooms and studies, as its properties can make it hard to relax and unwind.

* Orange: Stimulates hunger. Softer, lighter shades work great for living rooms, playrooms, and even bedrooms.

* Yellow: Inspires happiness and creativity. Use in any room to add brightness and vitality.

* Green: A healing and soothing color. A perfect choice for a living room or any room where you want to enhance the energy of balance.

* Blue: A healing and soothing color. A perfect choice for a living room or any room where you want to enhance the energy of balance.

* Indigo: Yet another color that heals and soothes. Good for a child’s room to promote healthy sleep habits.

* Purple: A highly spiritual color that promotes richness and transformation. Can be too high of a frequency for a large room, so consider using accents of purple as an alternative to painting an entire room.

Light. Light exerts a strong influence over how we feel. Take care to light each room so that you achieve the appropriate calming or energizing effect.

* Sunlight: Natural lighting is the very best way to brighten a room.

* Good Quality Lighting: Second best to natural lighting. If possible, don’t skimp when it comes to buying lamps, track lighting, or fixtures for your home.

* Bright Lighting: Promotes energy and activity (for a playroom, for example).

* Low Lighting: Promotes calmness and relaxation (for a bedroom or study, for example).

Include a warm body.

Bookshelves and books, a coffee table or end table with a small collection on it? Those wall racks for photos and pictures to lean against that Pottery Barn and other places sell would warm up the room. Think of things that reflect you and use them to personalize and cozy up the room. Couch cushions in colorful patterns would also warm things up.

Put a mirror on at least one wall. I have a mirror that I framed with a homemade needle work frame. My sister has one she used a mirror engraving kit on and it is beautiful.

Get a very nice throw rug at one of the furniture store closeouts in your area, they are usually on sale for under $50 and well worth the price.

artwork, something that can make you smile even on a drab day.

Experiment with lightbulbs in different wattage to figure out what looks comfortable and feels good.

Plants, artwork/prints on the walls, and throw pillows to bring in some color. Not sure if you have a rug on the floor? That can make a huge change, warms up a room.

Things that look touchable! Rugs on the floor, cashmere or fleece throw blankets over the side of sofas, etc.
Are your blinds of the white variety? Although they can be expensive to change out, blinds with a yellow or cream tint keep a room from looking like a hospital. Also, try layering window ‘art.’ Start with a heavy base drape in a thick material, and use complimentary color curtains over it in cotton or sheer (make sure to stick with the same scheme; is your furniture in jewel tones or earth tones? That kind of thing)
I’m not a fan of air fresheners, but essential oil burners instantly cozy-up a room. You can find cute models and mix and match essential or fragrant oils to create a scent that works for the space. It stays around longer, unlike scented candles, but isn’t as heavy as incense.
Plants are always a good idea for any room, but stick with things that won’t stress you out to take care of. For example, don’t purchase a weeping fig tree. African violets, diffenbacchia, bamboo, ivy, spider plants, and wax plants are all good choices.
Also, avoid overhead light! Find floor lamps with creamy shades that are easy to turn on with step-buttons. You can find a wide array at Ikea!