There's an art to it. The deeper the slits, the better.
When dressing, when you've about got everything positioned, unbutton one more time and then pull the shirt tail down hard in front of the holster and behind it one more time, and give it a little tug fore-and-aft to settle the material down into the slits. Then when you cinch your belt snug, it really traps the cloth in there and it shouldn't pull out.
Then adjust the "blousing" all around so that the natural folds of the shirt material envelop any bulges and put the belt and loops into the shadow of the shirt.
It's a bit exaggerated in my last picture there, but really, if you get everything set and then raise your arms over your head, bend over and touch your toes, and do a few side twists, you'll tug the shirt up just enough to blouse naturally.