By the username, I'm assuming you're female. If you decide to stick with jeans, invest in some that are a dark shade, are well-fitted (get them tailored if you can't find a pair that fits perfectly), and are in good shape. Make sure you buy better quality tops; it shows. Some unique detailing will help, too. Invest in a few unique, shorter blazers that you can take on and off when needed (us ladies are always cold in those damn offices and server rooms anyways).
I've invested a little in a couple of decently-made pants (i.e. wool pants with good drape). If you hunt around a designer outlet store, scan the sale racks at some of the nicer department stores, or visit someplace like Winners, you can pay a reasonable amount for a mid-range item. I have a pair of Anne Klein wool pants that have taken a lot of abuse, still look pretty good, and have a couple more years use in them. Good wool is actually pretty hardy.
Stash a pair of dress pants and shoes at work, in case of big wig clients or meetings.
Some of that general advice would apply to the guys, too. Buy nicer stuff, and keep it in good condition. If you typically don't wear business, store one 100% business outfit (including shoes and dark socks) at work. I've noticed a lot of the men's suit stores now sell semi-casual shoes, shirts, and higher-end jeans that are meant to be worn together, sometimes with a sport coat. That will make a much better impression than your worn-out shoes from Wal-mart, and the $15 - 50% polyester shirt you picked up from the mall. Impressions really do matter.