Play Mafia War

Don't fight early on in the game. just do jobs and build yourself up.

ONLY BUY MIKE'S.
each time you level up, put 1 pt in Atk, 1 pt in Def and the rest in Energy. you will always need more Energy.
Search Tagged for people with Mafia Wars as their name or keyword on the profile and add them. This will build up your ability to buy Mike's and get lots of cash rolling in.

when you buy Abandoned Lots, buy them 5 at a time or you'll get hit with a price increase. it's ok to have a few extra laying around for a new Mike's now and then.
if someone attacks you, don't worry about it. keeping your attack and your defense up at the same value will help.

Only buy enough weak weapons/armor/vehicles for job requirements.
Chainguns, Towncars and Body Armor are all you need to stock up on. the rule is... when you fight (or if someone attacks you) each person in your Mafia will get one of each item. when you read your attack results you'll see the strongest items you have listed first in decending order. if you have items showing up that are weaker than Chainguns, Towncars or Body Armor, buy a few more of those when you have the extra dough.

Eventually you'll have so much attack and defense that people will smack off of you like a wall, giving you money and xp to the point they will leave you alone.
make sure you finish the jobs of 1 level at a time. don't bounce around unless the job needs a special loot item to complete it. doing this makes sure you don't miss mastering up and getting special loot.
but definitely build up your friend's list. the bigger your Mafia number... the less likely someone is going to attack you... the more likely you'll have so much cash rolling in you won't have to worry about buying Towncars, Chainguns and Body Armor. Don't wait around watching your Energy fill up. GO HAVE A LIFE! the points won't fill up any faster with you watching them. Come back in an hour or more and do a few Jobs, then walk away!