Tips cityofthebigshoulders

From the rulebook:

Typically, stock value will increase during the course of the game. If given the option to either keep your personal money at the end of a Stock Phase, or the option to buy another share, you should almost always purchase a share even if that company is controlled by an opponent. That share will pay you money when that company operates and pays dividends. It will probably increase in value between Decades as well. Money in your hand will not do that. Thus, the better investment is to buy shares in something, even if it’s not the best company in the game. You can always sell it later.

It is often a good idea to buy all the resources from the $10 space, unless you want to prevent other companies from being able to purchase the resources they need to operate.