Practice practice. Its also mainly about the way you paint. Keep paint thin enough and dont overload the brush with paint by stroking off some on tissue.This way thev paint wont overflow onto another place. Do several layers until smooth and just correct mistakes by painting over. Tamiya/mr hobby pot paints arent really suited for handpainting. Those paints by miniature paint makers such as vallejo(the best!!!), gw citadel ,army painter etc dries smooth when handbrushed. You could read more on www.coolminiornot.com if you'd like.
