Finishing all the jobs in that stage: Associate, Street Thug (etc.)
By mastering jobs and winning fights.
The "prize" you get from mastering Boss tier in Mafia Wars is an iten which gives you double energy points: Every 3 minutes you get, instead of 1 point, 2 points of energy.
a 30 second reduction on energy regain
you will receive an M16 item that has a very high attack stats.
By doing the soldier job stage
You work through mastering the jobs, like in new york, cuba, and moscow before.
Che's Beret is a loot item for Mastering a Level in Cuba. You can only ever have 1 of these.
You receive Che's Berret after mastering level 3 El Padrino job tier in Cuba.
when you finish the underboss tier, you will receive a helicopter that decreases the energy timer by 30 seconds.
The firebomb in Mafia Wars by Zynga is obtained through the job STEAL A TANKER TRUCK at the soldier stage.
The Barsuk SUV can not be bought but could be gotten though mastering Baklany. http://mafiawars.wikia.com/wiki/Job_Tier_Mastery
Energy can be purchased from godfather points. Also mastering the boss tier and the consigliere tier will increase your rate of energy regen