Not sure what opinions might be on this, but I always wipe a little copper grease on the threads and the sealing washer before installing plugs (after cleaning off the seating surface at the top of the plug hole). And always, as JC suggests, start the plugs in by hand, to ensure they are properly threaded, before putting the socket on. Don't go gorilla mad with tightening either. Snug and a bit more should do. Further tip - do them one at a time, removing and replacing the spark plug leads as you go - that way, no way they can get mixed up. (You can even use some white paint / correcting fluid / paint marker to place 4 dots on the #4 lead, 3 on the #3 etc. to ensure they go back on correctly and never get mixed up again. )