I’m over 50, with a tummy and not much behind any more (except in the wrong places). But in the last few years I have started wearing NY & Co slacks, and absolutely love them. They sit right at or maybe 1/2" below my waist so they don’t reveal any underthings, they’re inexpensive but made from enough different fabrics that they don’t look too casual, they’re almost too long even for me, which is amazing since most of my height is in my legs.
But the key to all well-fitting clothes is: Tailoring! Tailoring, ladies! That doesn’t have to mean paying some lah-di-dah expensive place to custom-fit hand-made clothing; it can mean buying pants big enough for your hips/thighs/behind and taking them in elsewhere! In my case, I just trot out my sewing machine when something doesn’t fit and take in a bit here and there – it’s much easier most of the time to take in than let out – and all it costs me is the price of thread. You can buy a $20 pair of NY & Co slacks and have them tailored to fit your waist/ass/thighs for $10 and they’ll look terrific! I follow Stacy & Clinton’s dictum (of What Not To Wear) of fitting the biggest part of you, so if those pants are too tight? Go up a size and tailor down!
A few years ago whiterabbit needed a nice pair of black slacks. Besides being ultra-short, she also doesn’t have much of a waist. So we bought pants to fit her biggest part, her middle, and they swam on the rest of her – till we got them home. I took in the hips, hemmed the bottoms, and shortened the rise slightly, and for half an hour’s work, she had pants that made her look like she had an ass for the first time in her life. And I’ve had no training in tailoring – it’s really easy to do, promise!
Another good source for basic, decent-quality stuff is Land’s End. Granted, it’s conservatively styled, but they also do a fair amount of tailoring at little or no charge. It’s a good source for basics, although their offerings tend to be seasonal – right now they’re offering mostly summer casual slacks and capris which mostly don’t work well for the office, but during the winter they offer a nice assortment of dressier wools and cords and such.