Taking a sweatshirt and layering it over a collared shirt is one of my favorite outfit ideas. One problem is when the shirt underneath is a bit thick and the sweater on top becomes lumpy and unflattering.
In the pictures below the photo on the left is layering the sweater on top of solely the button down top, The photo on the right is a bit more form-fitting and all around flattering, and used the tips found below.
1. First layer put on you collared shirt
2. on top of the button-down shirt layer a tight tank-top or cameo, causing the button-down shirt to be held in place, and hugs to your body smoothing out any major lumps. Personally I think it is also more comfortable because the shirt below stays in place, and I don't end up wiggling around trying to shift it in place all day.
3. Finally layer a long sleeve shirt on top!