If you have a t-shirt that is too short for your liking, or just one that you want to jazz up, adding a band of coordinating fabric can do the trick in a short amount of time. While you could just add to the bottom, adding in the middle can help give an otherwise shapeless t-shirt a better look. This might especially be a good idea to revive a shapeless, stretched out t-shirt.
Start with your t-shirt and decide where you want the band to be, and cut straight across.
Now pick your fabric. Cut out a strip that is about 1" longer than your t-shirt (across) and twice as tall as you want the band to be (I wanted it to be about 2.5", so I cut a 5" tall band). Cut that in half across (so now I have 2 pieces, each is the length of the t-shirt cut and 2.5" tall). If your fabric is not directional you could just cut one band twice the length of your t-shirt at the height you want.
Sew the fabric into a loop. If it's directional, make sure the front and back pieces match up that way.
Turn your t-shirt pieces inside out. Turn your loop inside out. Pin the top of the loop to the top of the fabric and sew all the way around. Pin the bottom of the loop to the bottom half of the fabric and sew all the way around.
Turn your t-shirt right side out and you're done!