personal stats (Attack Defense) can be raised by using skill points obtained each time you level up. Your overall Mafia strength is calculated for only the first 501 members in your mafia. There is a WIKI article on how to calculate this. Robbing and fighting are the best way to obtain loot items. Each of these will count towards your overall strength. When you have more than 501 of 1 particular item, it is no longer counted. These are the items you can sell, gift or trade.