PENETRATING OIL, HEAT AND HARMONICS are your friends.. use them liberally.
another reason I've never broken off a bolt in good condition is because I use the right tools (for this I thank my father.. I'll forever remember him saying " bigger isn't better, just makes it easier to break things") and he's dead on correct.
Little bolts get little tools (1/4" drive) .. medium bolts get medium tools (1/2" drive) .. etc. Using only enough force to break the bolt loose.. without breaking it off. Leverage is your friend, but misused it will be your arch enemy.
The biggest factor is PRACTICE. After a while you can feel that point just before a bolt is going to let go or round off.. that is the point where you STOP and try another approach.
Sometimes, especially on old rusty beasts.. stuff is going to break, but if you try oil/heat/harmonics first you're likely to get out with minimal carnage.