The set up is the most important part of ironing t-shirt bodies. Begin by ironing any button flap and then the button area. Lay the shoulders flat on the board and smooth the material down toward yourself with the your hands. If you feel any wrinkles you can't see hold the underneath layer at the bottom and slowly pull while placing a hand on the top shoulder seam.

Start ironing by placing the iron next to the top seam and pulling it down sideways, again adding very little downward pressure.

This may cause a wave of material to precede the iron but this doesn't matter as you're pushing it off the board and it shouldn't crease.

Repeat that movement until you've done the top section and move up to the next section down. Smooth out from the center outwards and down at the same time. Spread your hands to get maximum feel of the cloth. Most t-shirts aren't square and trying to square them will lead to hours of fruitless attempts. It's best to accept that they won't hang straight after they've been worn and washed a few times and aim to get them "Flat" not "Square" to the seams.

Begin ironing from the center again moving out and down, rolling any fabric ahead of the iron.

Again, because you have not applied too much pressure you shouldn't iron in any creases. 

Move on again and smooth the fabric down. This time iron down sideways toward the bottom of the cloth. Once more this is to ensure that any roll of cloth doesn't cause a crease so keep light pressure on the iron. Note that hung t-shirts often get bulges on the shoulders because they are made of soft cotton and this warps.