With the exception of mattresses, I think furniture that's been around for awhile is far more interesting than the new stuff. And while you can choose to spend a fortune to decorate your home (and hey, the people who are contributing to the economy - and the future flow of great used stuff), you can also furnish on the cheap and creative by developing and discerning eye and spending some time looking.
Furniture thrift shopping is a bit different than looking for used clothes, which some of us are more familiar with. First of all, unlike clothes, when you are looking for used furniture, you are likely looking for a specific piece (and maybe even one with specific dimentions, that will fit in a spot you are looking to fill). So before you even start looking, have those measurements written down, as well as a swatch of the paint in the room and the other pieces already existent - taking a quick shot with your phone's camera will suffice.
Shop locally: Most communities have a high-volume store or stores, like Salvation Army or Goodwill, as well as smaller, usually (but not always) more expensive used furniture specialty shops or consignment boutiques. I think it's worth checking out at least three different stores, and keep in mind that depending on the location, merchandise can turn over daily (in NYC, San Fran or Boston) or weekly elsewhere. If you make a few pass-throughs and don't find what you like, consider buying directly from someone looking to sell a piece. Craigslist and Ebay (you can narrow your search to the local area on the latter site) are both great resources for people looking to get rid of great pieces and you can always ask for more images or measurements before you go check it out.
Check for issues: Once you have spotted a piece that you think might work for you, go over it carefully. Note any nicks or scratches, test for wobbliness, and look closely at wood - is it solid wood or some kind of cheap composite, meant to look like the real thing? If you can, move it around and look at the dresser/coffeetable/chair from various angles, making sure that the limbs of the thing are straight and unbowed. Is any fabric on the piece (chair cushions, for example) stained or faded? Will the color or tone of wood work with what you already have?
Envision changes: If you are planning on painting it, do a test where you imagine the furniture in your home, painted the color you have in mind. I do this by opening my eyes and getting a good look, then closing and imagining, then opening and looking again, then closing and imagining. If you spend a couple of minutes doing this, I have found you will get an accurate idea of how it could eventually look. Referring to the image of your space and other furniture here is smart too.
Look at the furniture's "bones":
I have a beautiful chaise longue and matching chair that were my grandmother's; I think the last time she had had the early 1900's-era pieces reupholstered was in 1968, so they were the worse for wear by the time I inherited them. Instead of spending money on a new piece, I ordered organic cotton canvas fabric from Rubie Green
and spent my money on redoing them (here's the before and after)
, and they are both showstoppers now. If you can find something inexpensively (or free on the street or from a relative), and it's essentially a great piece that just needs work, oftentimes just
Think creatively, especially if you're on a budget: Dressers bought on the cheap can take on a whole new life when painted and new knobs put on (I bought a dresser once for $10, and then spent about $50 on glossy yellow paint and new pulls from Anthropologie - it now looks like an expensive piece and I know it's one-of-a-kind). Tables with scratched tops but pretty legs can be covered with a series of fun, short tablecloths (think of the table in the Golden Girls' kitchen), and ugly, scratched shelves with simple lines just need a good coat of paint (use semi-gloss or gloss) - and to be filled with books - to look great. Headboards can be covered with draped fabric, and coffee table-tops can be decoupaged or painted with stripes and covered with a clear shellac.
If you aren't crafty, don't try to be: I love to convert furniture from garbage-worthy to glam, but not everyone is into it. If you consider a coat of paint a major DIY project, keep that in mind, and don't be tempted to pick up pieces that you will never put the time into to make great. You'll end up wasting your money, which is what this is all NOT about.