Mining isn't the most glamorous trade but it is probably the easiest way to make your millions. I posted this elsewhere but general tips...
Pristine metallic is pretty rare. Some kind folks have been compiling the known ones here
http://m.imgur.com/a/vEqgv#
When sellng then you always get a lot more if you take the mining missions. Worth swapping between solo and open to collect the missions you want. It doesn't take long to switch from solo to open or back again.
Sell at high tech stations.
Get allied with all the factions in the station you sell at so that you get access to more of the missions.
About half your cargo capacity of collector drones is usually enough. A bit more in the larger ships.
Tweak your sensor range (I think the default is PgUp PgDn keys) when you're in the asteroid field so that you can see where the next closest asteroid is but not so much that it clutters up the whole radar screen.
Work your way toward the planet so that you don't keep returning to the same, depleted, asteroid.
If you have a fragment targeted when you launch your limpet it will be programmed to collect only that fragment and then self destruct. You can target stuff while the limpets are working without causing them to self destruct.
Avoid mining elongated asteroids that are tumbling and rotating, it will kill your limpets.
If an asteroid is rotating use laser to hit either the extreme edge of the asteroid or hit the point around which the asteroid is turning. This will get the fragments clear of the asteroid and so safe to collect.