My strategy: don't level everything up together

1. Level up lemonade as much as you can (don't buy more unless you can afford an entire level or close to it at once)
2. Level up oil as much as you can
3. Level up banks as much as you can
4. Level up newspaper as much as you can
5. Level up everything else to the same level as newspaper
6. Level up everything on the right side (shrimp on) as much as you can

Most people suggest leveling everything up to 25, then everything to 50, and so on. I used to do this, but then I realized that until late stages of the game, lemonade makes the most money, then oil companies, then banks, and so on going backwards. Newspaper is the most expensive, but prioritize leveling it up because it adds a multiplier for other companies. Newspaper is your rate limiting factor. If you wait until you have enough money to upgrade newspaper before you upgrade anything else, things are going to move slowly.