I use a small ballpeen and a punch to try to shock the bolts. Rap on the bolt head a few times and then try again when putting pressure on the bolt.
Used this in conjunction with heat, penetrating oil and patience.
As frank said you will get a feel for it. When the bolt starts turning DON'T try to take it all the way out, Do like WS said and slowly work it back and forth. I do this fractions of an inch at a time. Yea it's gonna take you 5, 10 maybe even 20 minutes to work one bolt but, trust me, when you get that baby out intact it's worth it. ;D
I guarantee I would've broken every bolt I tried to remove in the merc if I hadn't used these techniques. Sometimes they're just not gonna come, this morning the welder called me up and said he worked on one bolt for 30 minutes and could not get it (He built up the bolt with stainless rod, tried removing it with vice grips, ended up welding a nut to the bolt and broke that off trying to remove it).
On to plan B: Drill out the bolts, try to save threads. If not, helicoil.
Lesson here?: Don't but a Merc with any broken bolts or signs of corrosion. :-/