It looks like you're specifically asking about how to prevent denim from giving you a muffin top -- by that I assume you mean jeans. Everybody's body is different, and two women who have a little extra around the waist will find that different things work for them, but here's what I do: I either wear a woven, structured shirt (untucked) with my jeans, or -- far more frequently! -- a relaxed and flexible knit top that I semi-tuck into my belt. Works perfectly. I find that linen or linen blends are particularly flattering because they collapse against the body, showing its shape, without clinging to anything. Stretchy tee-shirts are the worst for highlighting lumps, and boxy cotton tee shirts are generally unflattering unless they are a very lightweight knit.
I suspect higher-waisted jeans might help here, too. I can't wear them (they look pretty bad on me) but when I wear high-waisted skirts, it totally smooths over all waist lumps. Magic. so if your body shape can handle high-waisted jeans, they might be an option.
Also, sorry to contradict you here, Joy, but muffin top from childbearing may never go away. I am very slim and I still have extra around my middle, and some of the problem is actually loose skin that nothing short of surgery will take care of. But it's really not that big a deal