In new clothing, I sit up straighter, I keep my stomach in. No, it's not because I've found a stronger sense of pride in myself. Instead, it's to keep from everything hanging out. Clothing which fits – being neither too big nor too small – has no room for slouching. None whatsoever. Take this lovely sweater I'm wearing today. One momentary lapse of control and it won't be pretty.
So take it from me, new clothing will help you sit up straighter. It'll also work your abs, too!