Here you have some useful tips to help you increase your earnings:
 
EXPAND YOUR VAULT'S CAPACITY
 
The first thing you have to do to increase your income is to level up the vault in your firm. Don't forget to regularly check the vault and level it up until you get the max level. Moreover, check the Research list to run all the investigations related to this said room located on the bottom right part of your building. Remember: you should be able to complete the game without spending real money. If you see a price higher than your money limit, there should be a missed level out there! Read carefully the pending requirements in the vault's dialog window. 
 
UPGRADE YOUR OFFICES
 
All the upgrades you make in each room will affect your performance and efficiency, so you need to take care of every one of them so they can be harmonized and balanced and can function properly with all departments' help. You can check the upgrades or expansions available and their functions by tapping on each room.

These two tricks will serve as an example:

1) Wanna increase your documents' production? Then check the Paralegals' room  and expand it with more employees if it's possible. Also, don't forget about the delivery workers and room, as you may be producing a bunch of documents, but it will not be useful if you cannot deliver them to the corresponding lawyers! You need to check the whole process of a case since step one.

2) Wanna increase your number of clients? Check the lawyers offices that have angry clients and upgrade it as much as you can, so your prestige will rise and more clients paying more will come.

DO ALL THE RESEARCH

The upper-right microscope icon will be your best ally from now on! All these sections will take some time to be developed, but they will help you expand your firm. Check the boosters that they provide and choose the research according to your strategy. Some of them will increase your workers, others will bring more money, or will improve the efficiency of your lawyers, for example.

CHECK YOUR HIRED LAWYERS AND THEIR ATTRIBUTES

Take into account that your unassinged lawyers will charge you anyway, so if you are not interested in some, review your list of lawyers and consider firing them if they are not useful in these moments. This will reduce your expenses and help you continue with the game.

CHECK YOUR STATS

On the round graphic icon on the top right part of your screen, you can check with detail your economy. There, you can see if something is making you expend too much money or if you are not earning enough in some specific department.