First of all, "ART" is in quotes in the title of this post solely because I'm broadly defining it to mean any sort of framed wall decor (poster, hand-me-down, needlepoint, etc.) that you have, or may consider having, on your wall. It's NOT a snide way of indicating that these items aren't "real" art; I'm not elitist about the fine art world at ALL and my goal is always to help people spend wisely and to make their walls awesome.
That said, art that you have already quite possibly isn't living its best life right now; there are often ways to make it look better. Here are a few:
1) Re-framing. Framing, just like fashion, music, food, etc., is an industry with trends, innovation, and changes. When frames are dated, the art inside looks relatively worse. Re-framing every few years is one way to freshen up your art, and really your space. Black plastic frames from the 90s --- not so on trend these days. White lacquer or, bleached maple wood frames, big fat 8-ply white mats (since the old ones have probably turned yellow-ish): YES -- go all in as these will give your art a clean, contemporary and polished look.
2) Art height. Museums hang art with the center 57-60" above the floor. That number is generally considered a starting point in the home because furniture, architecture, and interior design sometimes warrant going higher or lower. Importantly, to make your pieces look coherent in a space, hang with the centers of all artworks at the same distance from the floor. For example: If your over-the-couch piece has a center at 62" above the couch, then the center of the piece on the adjacent wall should also be at 62". Again, there are exceptions, but this is a helpful guideline to making everything look better together.
3. Hardware. The hardware on the back of art actually matters. Wires, for example, are exceptionally annoying because the art will inevitably get crocked when there is one nail and a wire involved. My framers know to use D-rings on my jobs instead. D-rings are two rings (in the shape of a D, for "DUH") with one on each side of the back of an artwork. You put two nails in the wall, level and at the same width as the D-rings, and voila.....art that stays put. One bonus tip on this is that the D-rings should not show when the art is hung; you need the framer or artist to put them on the back of the art about 1/4 inches in from the edge so they don't stick out when the piece is hung. You could also do this yourself with a drill, but I'm not going to fully endorse busting out electric tools here unless you're a pro.
Re-curating people's current art collections is one of my favorite services we offer; it's economical and the results always exceed client expectations. I'm sorting through a bunch of before and after re-frames and will be editing and posting in the coming weeks.