I believe mining is only really profitable with a pristine ring (no Kharma, no

).
With RES just fly to a different part of the ring and you won't be bothered, but it's worth having shields as accidents can happen.
I've found the limpets to work quite well, don't target anything when using them as collectors (if you target something first it makes the limpet a one time only use), just fire the collector limpet at nothing then start mining, the limpet will scuttle around collecting stuff, after a while it will expire (the time depends on the quality of your controller, as does the distance in which they work).
Currently you have to monitor your refinery as it stops until you assign stuff to an empty hopper, but after 1.4 this should be automatic.
How many limpets you'll need depends on the quality of your controller and some experimentation, but I found I could mine 2 to 3 'roids per limpet.
But in reality mining, whilst strangely relaxing, is not particularly profitable and is very time consuming, it's worth trying at least once just to watch the limpets buzzing about, they seem oddly happy in their work.