1. Not considering how you live

Do you have kids? Hold frequent play dates? Host a weekly book club? Throw neighborhood dance parties? It might seem obvious, but you need to consider what you do in your living room and let that dictate the design.

And don't allow denial to creep in. Maybe you secretly want a glamorous, all-white living room that would fill a Kardashian with envy. But if your life is full of kids' toys and grape juice stains, you'll have to be honest with yourself about what works and plan around it. (Don't despair—there are ways to have nice things even when your kids are hell-bent on destroying them.)

2. Relying on a rough budget estimate

Home design and decor aren't usually cheap. Not only will you shell out for furniture and accessories, but if you're looking for any kind of expert help—from interior designers to contractors to electricians—that also costs money. This isn't the time for rough estimates. Make sure you've itemized every single potential cost ahead of time, and built up an emergency fund—because whatever can go wrong probably will.

"Have an idea of the number of pieces you need, the cost of them all, the cost of any labor you might need to hire—like an electrician to move a light, wallpaper installation, or a painter—and then take an honest look at the total," says Lori Paranjape of Redo Home + Design in Nashville, TN. "Once you know what the accurate numbers are, you can begin the project using the budget that you have."

3. Prioritizing style over comfort

Eclectic living room design
Eclectic living room design

Emily McCall

"I hate when people have tiny perchlike chairs taking up space just because it's an antique, or shallow, rigid sofas just for their look," says San Diego designer Christina Hoffmann. "The last thing I want to do at the end of the day or when I'm visiting someone's home is plop myself on an uncomfortably expensive-seeming piece of furniture."

Don't forget a place to put your feet up—figuratively and literally. Hoffmann recommends an upholstered ottoman or chaise (add bonus points if it also offers hidden storage space).

4. Forgetting to measure (and measure again)

Experts universally agree: Measure, measure, and then—just when you think you're done measuring—measure again. For precision, block off measurements of prospective new pieces with blue painter's tape. As a general rule of thumb, leave 18 inches between your sofa and coffee table, and a minimum of 32 to 36 inches of clearance between adjacent seating.

"Scale is a tough (and expensive) lesson to learn," Paranjape says. "There is no trick to this besides measuring everything ... twice!"

5. Choosing rugs that are too small

Contemporary home theater design
Contemporary home theater design

Moon Design + Build

Simply put, small rugs dwarf your space.

"Properly sizing a rug is a bit of an art form," Paranjape says. "It's important to ensure all legs of your furniture comfortably fit on the rug."

Opt for a larger, less expensive rug over a smaller, pricier one. Consider a jute rug for a nicely textured, earthy feel. For more visual interest, Chicago interior designer Claire Barnes recommends layering antique Oriental rugs (which she says instantly elevate existing furnishings) over a larger natural fiber rug,

6. Not thinking about lighting

Even well-designed living rooms can be uninviting without ample and thoughtful lighting.

"Lighting is one of the most important elements in a space and one that is often overlooked," says designer Jessica Davis of JL Design Nashville. Choose a table lamp (or hire an electrician to install an overhead pendant) in your favorite reading nook, and add floor lamps to anchor larger pieces of furniture such as the sofa.

Barnes recommends adding plug-in sconces next to furniture arrangements as an affordable alternative to the cost of rewiring a wall.

And "if you're building, don't forget to put a floor outlet in the middle of the flooring so you can float your furniture and still plug in a lamp on an end table," says Jaimee Rose of Jaimee Rose Interiors in Phoenix.

7. Not having enough seating

Modern living room design
Modern living room design

Dualchas Architects

Again, go back to your room's purpose: Will you need extra seating for football parties, ladies' night, or family gatherings? Plan accordingly by using space-saving options such as ottomans (group them in twos) or benches (preferably with built-in storage).

Chicago-based designer Summer Thornton likes to include accessory chairs around the perimeter of the room that can easily and quickly be pulled up to accommodate guests during parties.

8. Forgetting about wires

We love our technology, but the unsightly cords that come with it? Not so much. Luckily, there are some clever ways to hide those bad boys. Hoffmann is a huge fan of Ikea's cord and cable management kits, which help control cable chaos with tiny, easy-to-use clips and ties. If you're mounting your TV on the wall, be sure to find a contractor or handyman to snake the cord behind your wall so you aren't stuck looking at dangling cords. And talk to your cable provider about wireless TV receivers, which help you ditch most cords altogether.

9. Not containing your junk

Check out Ikea for a great selection of paper pop-up storage boxes for just a few bucks each; use these to contain everything from remote controls to cords to pet toys. When you choose furniture, consider pieces that serve multiple purposes: a coffee table with hidden storage to stow board games or stash kids' toys out of sight, or this fabulous side table from CB2 that doubles as a stool (genius!).

10. Overspending on the wrong pieces

Does that $1,200 side table really matter if you won't have any money left for other fine pieces? If your funds are limited, spend where it matters: the sofa.

"A beautiful sofa with plush, luxurious pillows makes all the difference in a living room," Hoffmann says. "Consider hitting up the fabric store and splurging on something great that expresses your style—then have a local seamstress sew it into pillow covers! You can use your existing inserts or get some on Amazon."

11. Stopping before you're truly finished

You've selected your couch, chairs, and coffee table, but don't forget about the essential finishing touches that will take the room from so-so to ah-mazing. Accessories such as throw blankets, decorative pillows, unique lamps, artwork, and decorative window treatments will give your space a textured, layered look.

"I find that people often stop when the room is about 80% complete," Thornton says. "They are so focused on the main pieces—the sofa, side chairs, and ottoman—and they think the space is done once those are in. But in reality, great rooms have a lot more layers and complexity."