There's a right way and a wrong way to wear slouchy clothing. The wrong way is by wearing two baggy items simultaneously. If you're going to keep your shirt looser, wear something fitted on the bottom. The same rule applies for flowy skirts or wide leg pants; show off your figure by wearing a body skimming shirt. When you wear something too billowy all over, you will repel all of the boys and give off the "I think I'm fat" vibe. Never good.
*Tuck the shirt in for a clean look, or leave it 1/2 untucked to say to people "I'm cool and can pull this off. Jealous?"