My kids wear hand-me-down and second hand T-shirts, pants and shorts. To keep them from looking like what they are, I iron (even the T-shirts) with store brand wrinkle release spray. The clothes end up looking crisp and brand new. I use an old pillow case to cover T-shirt decals to keep them from melting to the iron.
Then make sure to hang the T-shirts on hangers instead of folding them.