Clothing (button down shirt) question?

Ok so I have this button down shirt I want to wear today. It fits alright and the length of the shirt isnt so long, but the sides are really wide and its too big on my side/hip area and it just throws off the whole outfit. While trying the shirt on earlier I tried pinching an area in the back so that the front fit more snug around my stomach and it worked perfectly. But how do I keep it that way? Do I use a pin or something? What kind? Is there another method? Please help!