The Power of Flowers.
One of my favorite tricks when I'm looking to brighten up my apartment, is to add some fresh cut flowers around the place. I like to make a habit of heading over to our local San Francisco Flower Mart on Fridays, so I've got bouquets all weekend long, when I'm around to enjoy them and have friends coming by. The flower mart is good, because you can can get great flowers and plants at slightly higher than wholesale prices (unless you have a resale license, in which case you can buy them at wholesale prices!). So check around your city for the closest market. I'm a sunflower or gerber daisy kinda girl - they're such happy flowers and I can usually find a good bunch for under $10. Another one of my favorite spots for flowers is Trader Joe's, a neighborhood market that's popping up all over the country. I believe one just opened in NYC. They have super inexpensive bouquets starting at just $2.99! They also have live orchids and other small house plants, sometimes potted in some very nice containers.
Whether it's cut flowers, or live plants, both add a fresh look and a bit of color to your home. And live plants (I'm not talking crazy vines everywhere!) obviously last much longer, so they're even less expensive and get you more bang for your buck, so to speak. And growing fresh herbs is the best! Not only do they look great and smell so tranquil, but there's also a really good feeling you get when you trim off a sprig of your own rosemary to toss into your recipe. I don't know why, it's just gratifying and makes me smile. If you've had trouble growing your own perhaps check in with You Grow Girl for forums and advice.
Succulents are really cool, too (especially if you've got a black thumb and kill every orchid in sight, like I am prone to do). These modern, and yet slightly prehistoric looking species are quite resilient. They can withstand a lot of climate changes and a whole lot of neglect. I plant mine in funky containers - sometimes vintage pottery I've gotten at flea markets or thrift stores, and they look pretty cute (and also make nice hostess gifts). Flowers and plants, nature's little way of brightening your home on a budget.