Mansion
Make upgrading your mansion as often as possible your priority as many other building types can only be improved once your mansion has been upgraded.
Library
Don't forget to uprade the library as doing so allows you to research certain perks which benefit you in the long run such as train faster or make your soldiers work more efficiently in Combat.
VIP Points
Try to complete Daily Missions and Daily Family Missions as doing so will increase your VIP level as well as earn you Rewards.
Special Items
Although there is no need to hoard your special items and boosts you do need to use them at the appropriate time in order to get the most out of them.
Money Boosts
It is more beneficial to use these as soon as you get them instead of saving them as having your vaults full of money and goodies is risky.
Free Items
Every 20 minutes you can gain free items by tapping on the box next to Don Vito Corleone.
Log In Daily
Try to log in multiple times throughout the day as doing so will allow you to collect the resources on offer which you can then use to upgrade your buildings, perform research and attack.