I use an 18mm socket for the plugs (18 I think), and 2 inch extension and rachet. Once the plugs are loose, I take the socket off, and get a 12 inch piece of 3/8 inch fuel line and put that on the plug, and use that to twist the plug loose (because you can twist it to a 90 degree angle giving yourself lots of room)
For the new plug (after its gapped, and anti-seize) put the fuel line back on it, and reverse. The good thing about the fuel line, is that if it starts to cross thread ANY, the fuel line will spin, but the plug wont move anymore, so it wont tear anything up.
This is how I do it...and there might be better ways out there to do it.