Trench warfare
The process of gun registration would often alert the enemy an attack was being planned. Towards the end of 1917, artillery techniques were developed enabling fire to be delivered accurately without registration on the battlefield—the gun registration was done behind the lines then the pre-registered guns were brought up to the front for a surprise attack.

Mortars, which lobbed a shell in a high arc over a relatively short distance, were widely used in trench fighting for harassing the forward trenches, for cutting wire in preparation for a raid or attack, and for destroying dugouts, saps and other entrenchments. In 1914, the British fired a total of 545 mortar shells; in 1916, they fired over 6,500,000. Similarly, howitzers, which fire on a more direct arc than mortars, raised in number from over 1,000 shells in 1914, to over 4,500,000 in 1916. The smaller numerical difference in mortar rounds, as opposed to howitzer rounds, is presumed by many to be related to the expanded costs of manufacturing the larger and more resource intensive howitzer rounds.
The main British mortar was the Stokes, a precursor of the modern mortar. It was a light mortar, simple in operation, and capable of a rapid rate of fire by virtue of the propellant cartridge being attached to the base shell. To fire the Stokes mortar, the round was simply dropped into the tube, where the percussion cartridge was detonated when it struck the firing pin at the bottom of the barrel, thus being launched. The Germans used a range of mortars. The smallest were grenade-throwers ('Granatenwerfer') which fired the stick grenades which were commonly used. Their medium trench-mortars were called mine-throwers ('Minenwerfer'). The heavy mortar was called the 'Ladungswerfer', which threw "aerial torpedoes", containing a 200 lb (91 kg) charge to a range of 1,000 yd (910 m). The flight of the missile was so slow and leisurely that men on the receiving end could make some attempt to seek shelter.
Mortars had certain advantages over artillery such as being much more portable and the ability to fire without leaving the relative safety of trenches. Moreover, mortars were able to fire directly into the trenches, which was hard to do with artillery.

Strategy and tactics: The fundamental strategy of trench warfare in World War I was to defend one's own position strongly while trying to achieve a breakthrough into the enemy's rear. The effect was to end up in attrition, the process of progressively grinding down the opposition's resources until, ultimately, they are no longer able to wage war. This did not prevent the ambitious commander from pursuing the strategy of annihilation—the ideal of an offensive battle which produces victory in one decisive engagement.
The Commander in Chief of the British forces during most of World War I, General Douglas Haig, was constantly seeking a "breakthrough" which could then be exploited with cavalry divisions. His major trench offensives—the Somme in 1916 and Flanders in 1917—were conceived as breakthrough battles but both degenerated into costly attrition. The Germans actively pursued a strategy of attrition in the Battle of Verdun, the sole purpose of which was to "bleed the French Army white". At the same time the Allies needed to mount offensives in order to draw attention away from other hard-pressed areas of the line.

The popular image of a trench assault is of a wave of soldiers, bayonets fixed, going "over the top" and marching in a line across no man's land into a hail of enemy fire. This was the standard method early in the war; it was rarely successful.  More common was an attack at night from an advanced post in no man's land, having cut the barbed wire beforehand. In 1915, the Germans innovated with infiltration tactics where small groups of highly trained and well-equipped troops would attack vulnerable points and bypass strong points, driving deep into the rear areas. The distance they could advance was still limited by their ability to supply and communicate.
The role of artillery in an infantry attack was twofold. The first aim of a bombardment was to prepare the ground for an infantry assault, killing or demoralising the enemy garrison and destroying their defences. The duration of these initial bombardments varied, from seconds to days. Artillery bombardments prior to infantry assaults were often ineffective at destroying enemy defences, only serving to provide advance notice of an attack. The British bombardment that began the Battle of the Somme lasted eight days but did little damage to either the German barbed wire or their deep dug-outs, where defenders were able to wait out the bombardment in relative safety.
Once the guns stopped, the defenders had time to emerge and were usually ready for the attacking infantry. The second aim was to protect the attacking infantry by providing an impenetrable "barrage" or curtain of shells to prevent an enemy counter-attack. The first attempt at sophistication was the "lifting barrage" where the first objective of an attack was intensely bombarded for a period before the entire barrage "lifted" to fall on a second objective farther back. However, this usually expected too much of the infantry, and the usual outcome was that the barrage would outpace the attackers, leaving them without protection.
This resulted in the use of the "creeping barrage" which would lift more frequently but in smaller steps, sweeping the ground ahead and moving so slowly that the attackers could usually follow closely behind it. This became the standard method of attack from late 1916 onward. The main benefit of the barrage was suppression of the enemy rather than to cause casualties or material damage.
Capturing the objective was half the battle, but the battle was won only if the objective was held. The attacking force would have to advance with not only the weapons required to capture a trench but also the tools—sandbags, picks and shovels, barbed wire—to fortify and defend from counter-attack. A successful advance would take the attackers beyond the range of their own field artillery, making them vulnerable, and it took time to move guns up over broken ground. The Germans placed great emphasis on immediately counter-attacking to regain lost ground. This strategy cost them dearly in 1917 when the British started to limit their advances so as to be able to meet the anticipated counter-attack from a position of strength. Part of the British artillery was positioned close behind the original start line and took no part in the initial bombardment, so as to be ready to support later phases of the operation while other guns were moved up.
The Germans were the first to apply the concept of "defence in depth", where the front-line zone was hundreds of metres deep and contained a series of redoubts rather than a continuous trench. Each redoubt could provide supporting fire to its neighbours, and while the attackers had freedom of movement between the redoubts, they would be subjected to withering enfilade fire. They were also more willing than their opponents to make a strategic withdrawal to a superior prepared defensive position. The British eventually adopted a similar approach, but it was incompletely implemented when the Germans launched the 1918 Spring Offensive and proved disastrously ineffective. France, by contrast, relied on artillery and reserves, not entrenchment.

Life in the trenches: An individual unit's time in a front-line trench was usually brief; from as little as one day to as much as two weeks at a time before being relieved. The 31st Australian Battalion once spent 53 days in the line at Villers-Bretonneux, but such a duration was a rare exception. The 10th Battalion, CEF, averaged frontline tours of six days in 1915 and 1916.
The units who manned the frontline trenches the longest were the Portuguese Expeditionary Corps from Portugal stationed in Northern France; unlike the other allies the Portuguese couldn't rotate units from the front lines due to lack of reinforcements sent from Portugal, nor could they replace the depleted units that lost manpower due to the war of attrition. With this rate of casualties and no reinforcements forthcoming, most of the men were denied leave and had to serve long periods in the trenches with some units spending up to six consecutive months in the front line with little to no leave during that time.
On an individual level, a typical British soldier's year could be divided as follows:

15% front line
10% support line
30% reserve line
20% rest
25% other (hospital, travelling, leave, training courses, etc.)

Even when in the front line, the typical battalion would be called upon to engage in fighting only a handful of times a year; making an attack, defending against an attack or participating in a raid. The frequency of combat would increase for the units of the "elite" fighting divisions; on the Allied side, these were the British regular divisions, the Canadian Corps, the French XX Corps, and the Anzacs.

Some sectors of the front saw little activity throughout the war, making life in the trenches comparatively easy. When the I Anzac Corps first arrived in France in April 1916 after the evacuation of Gallipoli, they were sent to a relatively peaceful sector south of Armentières to "acclimatise". In contrast, some other sectors were in a perpetual state of violent activity. On the Western Front, Ypres was invariably hellish, especially for the British in the exposed, overlooked salient. However, even quiet sectors amassed daily casualties through sniper fire, artillery, disease, and poison gas.