I almost exclusively own high-waisted bottoms, which means that I'm always tucking my shirts on. It works out fine as long as the shirt is fairly fitted. However, in the case of most of my button-downs, tucking simply doesn't seem to work. So, I looked up a way to tuck in a button-down so that it would actually stay put.
Pre-DIY: see? Totally baggy!
Step 1: Unbutton pants and place the base of your shirt neatly downward. Your shirt should not be scrunched into your pants as they’ll easily come untucked.
Step 2: Pinch the shirt at the side-seams and pull tight to collect the excess fabric.
Step 3: Fold this excess fabric backwards. You want both back and front of your shirt to sit flush against your body; no bunching (you haven’t pinched enough fabric) nor pulling (you’ve pinched too much fabric).
Step 4: Button your pants up, tightly securing the folds against your body.
Step 5: Adjust if necessary to neaten the shirt.
Ta-da! Kind of better. I bet this would've worked better with either a thicker shirt or one that's a little more form-fitting. It did take a bit longer for the shirt to become baggy using this method, but it still became baggy eventually.