good method is to find a large area of native ores, let's say, cassiterite. once you find a large radius of ores in an area (you should be marking them with cobble once you find it), go to the center of the circle and start digging a 3x3 shaft down a few layers and branch out for prospecting if you need to. once you find samples, start digging that way, up and down. the end result should be tons of metal.
hope this helps