Corp value is related to profitability, and the PE will determine how fast the value increases or decreases. You can also impact the value of a corp, if it is publicly traded with stock buy and sell orders.
Also, some corps are only occasionally profitable, so they will goo up for a while before falling again. When building corp, focus on high value consumables. Avoid things that can just sit in inventories and never get used.
So what you need to do is experiment with the settings on the corporations page for trade strategies, salaries, supplies, etc. Many people will just tell you a set of numbers to plug in, but then you will not learn how the different items interact to produce profitable corps.