Besides field fortifications, infantry had a few basic techniques to mitigate the worst of artillery fire.

Probably the simplest was laying on the ground; this made the unit in question a much smaller target, though it was often difficult to get the men off the ground when the time came to advance. Another method was positioning the infantry behind some kind of natural cover, such as a hill or ridge. At this time, there were no 'forward observers' who could quickly and reliably relay target information to the guns, so the gunners had to pick their own targets from the position of their battery. Artillery fire relying as it did on direct fire, blocking the gunners' line of sight significantly reduced artillery's effectiveness.

It's also important to keep in mind the distances involved. By the later Napoleonic Wars, when the mass use of skirmishers was basically universal, the infantry lines* were usually stationed a considerable distance apart; each line would be something like two hundred meters behind their skirmishers, which were separated by perhaps another hundred meters from each other. With the artillery usually in line with the infantry or some distance behind it, the tight formations of the enemy were usually well outside any kind of 'can't miss' range.

At 600 yards, a line of men is only about .2 degrees tall in geometric terms. As a result, artillery often had to make several ranging shots to determine exactly how high to elevate their guns. Knowing this, infantry could mitigate damage by waiting until a ranging shot struck a man, then immediately advancing or retiring some distance -say 50 yards- to throw off the enemy's ranging and buy more time. They would wait until another ranging shot hit the line, then repeat the process.

*nb it was typical for a line of infantry to be mostly empty space at this time; while waiting for orders, most infantry were kept in columns separated by intervals equal to their frontage in line. Depending on the situation, they might deploy in line to keep artillery fire from plowing through too many men while holding their position.