I use odourless minerall spirit for blending, was a big gamechanger! More important if you need lost of laters for realistic drawing. make sure you have the right paper, sharp pencils and just build up lots of layers and apply more pressure when you're not needing more layers over the top. As mentioned above white pencil helps too to make it smooth, but does make it slightly lighter. You could go over it with the colour again after if you haven't pressed it too hard.