need brake caliper broken bolt help

[blue420]

i snapped off the lower left caliper bolt changing the rotors on my 86 420 sel, i drilled out the bolt and then actually broke off the easy out, in the hole, not good- i have the hole cleared now but will need to tap it, -- any hints on how not to continue to mess this up? thanks, i am taking the rotors to a dealer to remove the hub- those hex bolts have disaster writen all over them.