1. Don't buy furniture in sets. I don't know many people who want their home to look like it came out of a catalog. Most people want their home to be a reflection of themselves and their personality. I think this is best achieved by buying individual furniture pieces instead of sets.
2. Hang your window treatments high. I feel like this tip is everywhere in Pinterest, but in case you aren't aware of this, here's a great image.
Hanging your window treatments higher makes your windows and ceilings look taller.
3. Mix and match styles. Just because you have a mid-century sofa doesn't mean that you can't mix in traditional furniture with it.
4. Neutral walls and colorful furniture OR colorful walls and neutral furniture. I recommend one or the other, not both. There are a few exceptions to this rule - Holly from in the fun lane does a great job with the neutral on neutral color scheme and Emily Henderson did a very colorful tween girl's room - but I think this near to impossible for the regular person to pull off.
Everything in this photo (except the candle) was bought at a thrift store.