You should also use some penetrating lubricant. Some are marketed specifically for penetrating and loosening threads. I don't recall if you can or can't get some into the threads from the back, probably not. Not much of the lube might get past the seat of the bolt but you need every small thing on your side. Also make sure you buy a good quality socket that won't break (Snap-on is one of the best). You can also try some very strong blows to the bolt with a hardened steel punch (then back to the socket and breaker bar extension as noted above). An air impact wrench can also be a big help. Never use these for tghtening wheel lugs but their impact capability is just what is needed to loosed stubborn bolts. If the bolt does break you can probably extract it yourself. Try to get all of the other bolts and wheel off. Then try the following:
1. Much more penetrating lube.
2. Use an extraction tool (
http://www.mcmaster.com/#screw-extractors/=a2tbqz). Drill a hole with a cobalt or carbide drill bit (special bits for strong/hard metals) then force feed the reverse threaded extraction bit into the hole. It then becomes locked into to the broken bolt and can apple torque to loosen it.
3. Heat from a torch will work wonders on really stuck fasteners. Heat up the broken stud with the heat as concentrated on the bolt as possible. get it really hot but don't get any parts red hot. After heating get right to the extraction tool
If all of this is way too much, just take it to any good mechanic. Don't let a hack ruin parts on your car though as that is somewhat likely in cases like this.