How did WWI change the method by which wars are fought?

WWI, at least on the Western Front in France and Belgium, became a war of attrition to be fought in trenches.  Machine guns and barbed wire made frontal assaults suicidal and casualty figures for battles such as the Somme and Verdun were staggering.  Generals at the time still believed that battles would be won with cavalry charges and quick infantry maneuvers, but their ideas were proven horribly wrong by the end of 1914.  In order to break the stalemate on the Western Front, both sides looked to chemistry and engineering with the use of poison gas and tanks, respectively.  These proved to be indiscriminate killers of men and soldiers quickly lost any romantic notion of war they may have had.  

The airplane was also used in this war.  For the first time, cities were bombed from the air, though not to the extent they would be in the next world war.  Pilots were "guinea pigs" in new combat techniques and plane design, and more pilots died in accidents than in air combat.  

Finally, naval warfare was different as well.  Both Allied and Central Powers built up impressive armadas of giant battleships, thinking that there would be a massive battle for control of the seas.  Other than Jutland, which was inconclusive anyway, this never happened.  German submarines came closer to ending the war than any battleship fleet.  German submarines also played a key role in bringing America into the war.  

The most tragic change of this war would be the manner in which civilians and their goods were targeted.  Germany developed the Paris gun which could lob a shell over ten miles into the city.  While warfare against civilians happened before this war, technology now sanitized the process of killing a noncombatant.  Blimps dropped bombs on civilians.  They were considered vital to the war effort as they could be drafted or work in war industries, so civilians were considered fair game.  Both the Allied blockade of the North Sea and the German U-boat blockade affected food coming into Europe, but the Allied blockade of Germany proved to be more effective, and the winter of 1917-1918 was known as the "Turnip Winter" as Germans were reduced to eating a root otherwise reserved for their livestock.  The German army was not defeated in the field, but the German government was overthrown by a starving populace.  The Allies kept up the blockade throughout peace negotiations, and the Treaty of Versailles was signed under this duress, which made a war of retaliation likely.