Faux painting of large Palms in the corners with a darker grey/green and a muted yellow highlight around their edges to give them depth. I did this for my sisters living room and it actually made it look bigger.
Use comparable colors to livein' a room such as rusts, golds, yellows, deep reds in accent pillows, area rugs, plants or fake colorful flowered tropical style plants or if you have a country look going local colors such as yellows in daisies, sunflowers, blue bells, poppies, ect. Add different shaped mirrors to enlarge a room. If you are a modernist/industrial style, and are trying to add natural elements, rocks, quarts, colored glass bottles or decorations work well. Adding vine plants, fake or real, in empty area or very large pieces of furniture or architectual pieces makes them blend into the area they sit and makes them softer.
There is a website you can go to actually work with colors and accents for your rooms using your own pics or those in their program, it's from Behr paints, www.behr.com. I have recorated every room in my house this way over the last year with less than $200 each room. Some of the stuff I found off of freecycle.com for my area and you can actually join the one for the areas that are close to you. I would get something from someone that may not have been in perfect shape and just repainted it or fixed it or both and voila'! Definitely check out the website and you can play with different colors to match your wall color or even pick out on shade lighter or darker for just one wall to add depth or to make one area a more vibrant focal point. Just add you into it and you will be pleased with the results.