Two ways, one obvious, one not so obvious.
First, the obvious. More ratings = more profit. War keeps people tuned into the television and reading newspapers and news magazines.
Now, for the "not so obvious". Look up the names of the 8 or 9 large corporations that own the mass media outlets (I'll call them the "media giants").
For each media giant, find out who the major shareholders are.
For each media giant and each of their major shareholders, find out what other companies are in the portfolios (as major shareholders or controlling interest).
Then for each of those other companies (I'll call them "subsidiaries"), learn
1. Are they suppliers to the government or military, or to companies that are suppliers to the government or military? If so, what products and services to they supply,
-as a direct result of the war
-as a result of the reconstruction effort
-as a result of the war on terror
2. what services and/or products do they provide, not directly to the government, but for which price and/or demand increases as a result of the war or war on terror. For this also look at products and services that are likely to be in demand in the future as a result of the war - possibly nuclear energy??
(and for each subsidiary company of the subsidiaries, repeat steps 1 and 2. Keep going until you get to the bottom of the tree). You will be busy.
While researching, do a cross match between names on Boards of Directors and the Council on Foreign Relations to find the power motive. The CFR is much more than a "think-tank". CFR foreign policy becomes government foreign policy. Connect the dots.
As a hint, start with General Electric. GE is a Pentagon supplier. GE has controlling interests in various mainstream media outlets. GE owns other companies that meet or more of the three criteria listed under point 1 above.
GE's media outlets were among the main stream media outlets that colluded with the Pentagon to sell us the Iraq war. That cat is now out of the bag.
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If you start on that research, it won't be long before you conclude that centralization of media ownership is a grave threat to democracy in America.
Corporate influence over media coverage has always been a problem to some degree because advertising funds media.
What changed the stage drammatically was the change in regulation that permitted the centralization of media ownership into the hands of a relative few (8 or 9) corporations.
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EDIT
If you will permit me to stray a little off topic, there is one pitch I would like to make.
As a conservative, I do not see regulation as a solution. Rather, I see EDUCATION as the solution. If people become educated in "whys" and "hows" the main stream media goes about its propaganda dissemination, people should do what they always do when a product is flawed: STOP PURCHASING THE PRODUCT.
If there was ever a time for the public to turn off the television COMPLETELY (deny them their advertising revenue), and stop purchasing major newspapers and news magazines, NOW is the time. What do companies do when flawed products fail? They either correct the flaws or they go out of business. That is the free market in action. The beauty of the free market is that PEOPLE, the consumers, determine whether a product succeeds of fails.
That is the only way we can force the main stream media to report fully and accurately in the interests of the people. It will take at least half of us to act to force the necessary change. There are plenty of independent news organizations on the internet for people to keep abreast of world and local events.
THIS IS URGENT. As I write this answer, the main stream media continues to feed propaganda aimed a inducing support for what appears to be inevitable war with Iran. The pace of the propaganda will pick up as the war gets closer. The media is not giving us the straight goods about Iran. For example, the statements of Ahmadinejad as to "wiping Israel off the map" have been mis-translated, sensationalized and taken out of context. The media outlets know that, yet they continue to repeat the slanted version over and over again. Further, the Pentagon has admitted that they cannot prove many of the claims about Iraq supplying weapons to insurgents in Iraq. The media knows that too, but you rarely hear it. Iraq's nuclear program is being misrepresented to the American people as well. Anyone who wishes to spend some time looking, will find these facts verified by numerous independent news sources.
The corporate controlled main stream media is simply parroting the government propaganda as they did to lead us into the Iraq war. Will we buy it again, or have we had enough?
The wars with Iran and Iraq were planned years ago, before 9/11. The power-players behind the wars were not secretive about their geopolitical objectives for the region. The plan is now unfolding.
A "9/11-like" event was well documented as necessary to acquire public support to move the war plan forward and to scare us into giving up civil rights. That is a topic for another question, so I'll end it there.