Use neutral, warm colors on most walls. Let these be the background for your more decorative items, such as art, fabrics, or an accent wall. Personal favorites in the beige-gray family include HC-83 'grant beige' with accent color HC-86 'kingsport gray'; in the bluish-gray family, try HC-171 'wickham gray' with accent color HC-165 'boothbay gray' (all part of the Historical Color collection in the Benjamin Moore "Classic Colors" line).
Use the same color for trim and walls. Rather than calling attention to baseboard or window trim, create cleaner lines and a greater sense of space by using the same color on each surface. On walls, a matte or (washable) flat will provide the softest-looking finish and hide the most flaws. On trim, provide a little sparkle with a semi-gloss that matches the wall color. For interior doors, select a semi-gloss color one or two shades deeper for a bit more overall depth to the room.
Select furniture with clean lines in neutral, warm colors. Similar to #1 above, pick simple pieces that can stand the test of time, but may be dressed up with decorative pieces that are easily changed. Whether you lean more traditional, such as West Elm's upholstered Chester sofa, or more modern, such as Room & Board's Hayes sofa, pick something comfortable, then dress it up with some cool throw pillows or a pretty blanket. (Sometimes when I need a change, I create a new look by mixing up pillows from various pieces I already have.)
Take into account how you'll use a space and make sure the furniture fits. As enticing as a huge sectional may be, make sure it's what you need. For a media room used mainly for watching TV, a sectional might be just the ticket—one streamlined piece that accommodates everyone in the family. For a more formal living room where you entertain friends, you might consider a sofa and a couple club chairs. Ottomans and cubes are also great options, especially for small spaces where furniture can do double duty as an extra chair or table.
Whatever the configuration, be sure to select pieces that fit the scale of the room. For smaller spaces, consider furniture with low arms and visible legs—they feel less solid and create a feeling of more space. Also consider a loveseat, rather than a sofa—it's not often that more than 2 people will use it at a time anyway. A sectional can work nicely, as long it's configured specifically to work with the flow in your room—just keep in mind they're less flexible if you ever want to rearrange your room or use elsewhere.
Create a variety of lighting levels. This creates visual interest and ensures you have the light where you need it. Ambient lighting, such as recessed ceiling cans, provides overall light so you can see your way around a room. Task lighting, such as a table lamp, provides brighter light where it's needed to complete a task. Accent lighting, such as spotlights on tracks, highlights art or objects which require special attention. Switch up your look from time to time with new lamp shades or adjusting accent lighting to highlight a different art wall. Whenever possible, put all switches on dimmers so you can control the amount of light and further affect the look in the room.
Don't be afraid to throw in a few surprises! With a neutral background palette, you can afford to paint one wall your favorite purple color or use that sequined pillow you found on your travels. All the simple lines and muted colors are now in place, so reflect your unique style with a few things you really love!