Almost any pair of jeans can be dressed up if done correctly but it's a job most easily accomplished with a pair of crispy, clean, slim, straight dark-washed denim. The less embellishment in the denim -- even down to no contrast stitching -- the easier it will be to pull off dressing them up. When sprucing up denim (of any wash), think of your jeans as you would a pair of pants -- simple, tailored, and worn with a slightly shortened hem that hits just at the top of the back of your shoe, or at mid-ankle. You can also get away with a good, clean roll to this length.
Depending on your personal style or destination, you can sport dark denim with a number of styles. Make the look lean a bit forward by peering backward -- pair them with the top of a slim three-piece suit, tie, and even a pocket square. More conservatively, you can just wear an Oxford shirt with a tailored sports jacket and (optional) tie. Either way, fitted sports jackets with slightly rugged (rather than ultra-smooth) textures pair best with denim. Try light wool for spring and summer and a tweed for the cooler months. Stick to neutral tones (black and white not so much) or monochromatic colors, have fun with patterns, and finish both looks with a pair of loafers.
As long as you stick with a tailored fit, now more than ever you can also dress up a pair of colored denim. From pink and orange to green, just be sure you keep the tone of the color as muted, soft, and tame as possible. Juxtapose the flair of your colored denim with a strictly conservative or preppy dressed up top -- navy blazer, plain knit tie, loafers, and a simple oxford shirt. You can get funky by wearing a striped knit tie with a plain oxford or the other way around—a banker-striped oxford with a solid tie. The same suggestions apply to white.
Lighter washes or an old pair of faded jeans (even some with rips) can also be gussied up with sophistication if done correctly. The main challenge here is getting your beat-up jeans to fit in a way that looks smart rather than slouchy. Any denim that is too loose fitting, baggy, or hemmed too long won’t look dressy. But once you’ve got the slim, straight with hem at the ankle fit (consider a trip to the tailor), casual coolness is but a few short steps away. Pair them with a darker, textured blazer and solid oxford shirt, and to up the casual-cool factor, opt for a pocket square over a tie. Throw on some suede slippers to be modern, or rock it with a pair of classic boat shoes -- no socks -- and suddenly you’ve breathed new life into your old pair of jeans.