Go slowly when you shop. Quality time in the fitting room—45 minutes to an hour—pays off. Pick a store with lots of inventory.
Sit or squat while you're trying on jeans to make sure not too much butt is showing (and to gauge comfort level).
Be flexible about size. An 8 in one brand may be more like a 10 (or a 6) in another. When in doubt, take more than one size into the dressing room.
Buy two pairs when you find great jeans—one to hem for heels, the other to hem for flats. You'd be surprised how fast styles get discontinued.
Tone down to a dark blue wash; it's most slimming. Special fabric treatments (whiskering, abraded texture) make sense only on areas of the body you want to spotlight.
Think wardrobe, rather than one grubby pair you wear to death (that's so college). Three to five jeans are the minimum: one or two for work or going out (dark, straight-legged denim looks most polished), one or two for weekends (with sneakers or flats), and one with extra give or stretch for "fat" days.
See a tailor if jeans aren't precisely right (but never buy them too small, or there won't be enough fabric to work with). Some stores—Banana Republic, Club Monaco—offer free alterations; Nordstrom does only basic hems gratis.
Prevent fade-out by washing dark jeans inside out in cold water. Dry-clean your dress-up jeans (the ones you wear with heels) from time to time: It's pricey, but you'll look crisp and immaculate.
Shop online if there are no local stores you like—it can be tricky, but good websites help. Shopbop.com has an interactive guide that shows you different styles on four figure types; at landsend.com you can see how jeans fit on a virtual model of your body.