I used to drill a line of 1/8" holes, close to each other, across the head of the plug, then Dremel them together into a slot, and use one of those 1/2" socket drive screwdriver bits (this came long ago from a jumble and has the MoD arrow to say it's a good'un) to fit the slot and undo the plug. As several have said (the hailwood site mentioned has the most subtle description, I think) drilling out the centre punchings carefully makes things easier.
Nowadays I'm fortunate to have a Bridgeport mill, so with a bit of jiggery pokery I can get access to the plugs with a 1/8" end mill and make a slot for the screwdriver bit more easily.
Loctite do a viscous, treacly, thread sealant (number 577) which is gap-filling and is ace at locking replacement plugs in and preventing oil leaks down the threads (I've always found that when screwed in flush, the plugs still aren't that tight, so this sealant gives peace of mind). HTH