Rowing machine and plank pushups.
Also, (this is the continually harder part) cutting out a lot of carbs and processed sugars from my diet (and the slightly easier part) eating more proteins and veggies.
I always HAD a firm stomach, you just couldn’t see it - I called it my ‘stealth abs.’
I could always do pull-ups and push-ups, and hundreds of reps of crunches - you just never would guess that from my slightly woobly midsection.
The rowing machine works really well, because in addition to working the stomach muscles, it’s also working back, shoulders, chest, and sides. All that tightens up my posture and makes me look thinner in the stomach because I’m not all slouchy.
In addition, the cardio aspect of it helps keep my fat burnt off, which has always been the real challenge for me. I think that’s the main reason I’ve finally tightened up visibly.
Push-ups are roughly the same idea. I put my feet up on a box (I’ve tried a balance ball, but my balance is shit, so I usually use that when I feel like concentrating on not falling, rather than actually doing any push-ups.) I got a set of those little handle things to push from, so my wrists don’t take too much abuse.
What I really noticed from the push-ups (started with them first because the rowing machine took a while to save up for) is that even without my stomach being much tighter visually, I looked thinner there because my shoulders and chest were broader and more squared off - in contrast with my newly wide shoulders, everything else looked slimmer.