How to increase your income without going crazy:
Write down how much cash you have.
Buy a Purple house.
When your cash recovers to the amount you have written down buy a Blue house.
When your cash recovers to the amount you have written down buy a Pink house.
When your cash recovers to the amount you have written down buy a White house.
When your cash recovers to the amount you have written down buy an Orange house.
Repeat these actions until all the color houses have reached their maximum cost.
Now pick the one color house that has a payout time that's most convenient for you to be able to tap EVERY TIME it pays out. For me that's White - 12 hour payout, twice a day.
Continue building that color house until you have about 300 to 400 of them - clear an area so you can put them all together - that way you can tap them all out quickly.
Once you have "enough" your cash will start building up and you can start buying all the land, or KEM farming, or Rat Trap Trucking, or Bloodmobile trucking, or what ever pleases you.
There will come a point when you can sell all the other color houses. I haven't gotten there yet because my multiplier isn't high enough so I keep spending my cash reserve on Rat Trap Trucks for donuts.
Have fun. Enjoy yourself (NO! Not like that! That's not what I meant! For God's sake!)