get the engine really really warm before doing so then basically if they feel like they are going to snap then stop before they do! they dont sieze on the threads usually, they tend to sieze on the tapered seat where they seal due to carbon build up. if you do snap one either live with it snapped or remove the head and get it machined out its not worth the risk trying to drill them in situ as you will no doubt get swarf in the engine and total it. i did a transit connect where i had to drill some snapped injectors out and was convinced i was carefull enough to prevents bits going into the engine but could not bring myself to start it up so took the head off and i was right there was shit loads of swarf there! lol