From Wintermute, the fastest farmer we know:
Want to break a lot of blocks fast and not even use your hands? For solid blocks, clear out a line in your world from one side to the other. Lay down the blocks until you're up against the wall. Put something small and heavy (like a wheel lock or socket) on the D key and lay something heavy and steady on your space bar (a cellphone works great!) and your Pixelian will harvest all those blocks for you while you go take a break!
How about background blocks, water, and lava?? For blocks you can run through or can get hurt bumping into, he has another great tip, though this one requires you to do a little work: Press the left button on your mouse and alternate between pressing the C and Z keys with the other hand as the blocks get broken. This tip helps those blocks disappear quick!
