Another really great way to mine is the mining laser (srsly? I'm the first to mention it?). Get yourself close to bedrock, set it to long-range, and point up. If you run into low lava block it off and try somewhere else, or if it's high enough build a quick catch for it so it's not a mess. If you have the resources for a quarry, you probably have the resources for a mining laser and plenty of EU. I sometimes like this method because it's fast and it's something I'm doing personally, because sometimes letting a machine do the work for you just isn't what you're after. Well it's probably somewhere in the middle, actually.
A really great supplement to mining by hand is wands. A wand of excavation for accross-the-ravine mining, and a wand of equal trade for ore nodes. Make sure to bring tons of grass/ferns and replace the ore with those blocks, since it'll get everything instead of just surface blocks. Go ahead and start off with a handful of each wand and try enchanting them all. Once you get a repair (or is it charging?) wand, the rest that aren't repair will just be the first ones you use up and the repair wand is the last one you need to make, if you keep it out of lava.
I really like the Steve's Carts mining cart. If fully upgraded it makes a MASSIVE tunnel with a track running down the middle of it. With a little attention you can guide it to make tunnels that turn, change elevation, and do other things that in the end give you a network of awesome subterranean tunnels connecting caves and such, already set up with a railway, a lot like mining tunnels found in the world. Yeah, you can do something like that with turtles too, and both involve a learning curve, and both give you a real sense of accomplishment when you pull it off well. I vote for the cart because I have more fun doing it. YMMV
