1. Found art. There is no rule that says that art has to be purchased. Magazine tear sheets take on a whole new life when they're collaged. Or carefully trim pictures from old calendars - instant art.
I like walls covered in unframed art, but put a frame on even the most lowbrow piece and you've got a whole new look.
2. Hack it. New furniture can be expensive, but if it's from IKEA, it's probably not. Of course, IKEA furniture doesn't have to look like it's cheap. This Fira mini chest was dressed up with decorative trim from Home Depot and a little paint to look like a much pricier side table from Anthropologie.
On Nine by Design, the Novogratzs are always finding cool stuff similar to this mirror at garage sales and consignment shops. They add a coat of spray paint and some varnish and end up with a masterpiece. A cheap one.
4. Put a pillow on it. Throw pillows can totally change the feel of the room, and if you know where to look (Etsy), they can be very affordable.
5. Little candles everywhere. Everything looks better by candlelight - and while big candles can get pricey, little ones are not. Stock up at IKEA or Pier One and change color as the mood strikes.
6. Cover it. Most cities have at least one good fabric outlet - in Baltimore, ours is called DeBois Textiles. It's a perfect place to buy a big, cool piece of fabric that can easily be turned into a great tablecloth. And I'm not even talking about having one made. Or sewing. I'd just cut the edges with pinking shears and be done with it.
7. Flowers. Flowers brighten my day like you would not believe - they spruce up a room like nothing else. They can be pricey, but they don't have to be. Trader Joe's has amazing deals on flowers, especially those in season. Hit up the store for a few bouquets and your house will feel brand new - for a few days, at least.