Stuck / Broken Bolt

What started out to be a simple repair has turned into a week long project. Started out to change my impellor on my 1989 Suzuki 175 HP OB, removed the first two bolts from the impellor housing, the next two bolts snapped off with low torque ???, man was I pissed. I had sprayed them with WD-40 previously. I guess WD-40 is useless for frozen bolts. I bought a can of PB Blaster and soaked the bolts stubs for 3 days, got a good grip with vice grips but still could not break them loose, ended up buggering the outside of the stubs. I did not want to heat the lower unit housing because I was afraid of cracking it. I soaked the stubs with Oil of Wintergreen (an old timer at work said this stuff works every time, just be careful because it is poisonous). I drilled a center hole and tried an easy out, got a good grip but could not break the damm bolt loose, ended up breaking the easy out inside of the drilled hole. To make a long story short, I had to take the lower unit to a machine shop to get the damm bolt stubs machined out. Luckily, a friend of mine work there and is doing it for no charge. I should be getting it back this weekend. Does anyone have any suggestions or stories on how to prevent this type of cluster f#&@ from happening again.