Only butter the side of the bread that will be face down cooking. You can butter the other side after the first side has cooked and it is ready to flip. This will prevent the butter from sticking to the foil covered brick or pan you use to press the Panini.
Make sure the pan is hot before you begin to grill your Panini.
Sliced bakery bread, focaccia, or sliced Ciabatta bread makes delicious panini’s and holds up well to the pressing. Regular white bread can become too thin and fall apart when being pressed.