Personnel in the open are most vulnerable to the following, in decreasing order of effectiveness:
Going prone has little effect against air burst or richochet fire, since the shrapnel is spraying downwards from above, but would cut casualties by 2/3 against quick-fuzed fire. 
Rolling terrain makes quick-fuzed fire about half as effective as flat terrain by providing natural cover, though it has little effect on airbursts. 
Short bursts at irregular intervals have a cumulative effect on morale. Surprise is essential if casualties are the objective. In general, the first few minutes of artillery can inflict a lot of casualties; after that, the main effect is to pin enemy forces or restrict their options.
Against troops in shallow trenches, airbursts can be effective; richochets usually don't have the right angles to penetrate the trenches; and impact-fuzed fire is ineffective.
Overall, though, it is very hard to cause casualties against dug-in infantry with artillery fire -- its main effect is to keep their heads down and prevent them from moving out. In games, this should result in artillery having no effect against personnel in trenches except for pinning.
White Phosphorous can be effective in driving personnel out of foxholes where HE can be used effectively.
The US Field Artillery Gunnery manual  states artillery is ineffective against minefields and wire, and using it thus is just squandering ammunition. Minefields are frequently more difficult to neutralize after the ground has been stirred up by artillery fire.
Wesely  and the USSR Manual  disagree, saying fire against both can be effective if enough ammunition is expended.
Precision fire is used against guns. If under cover, neutralization is the goal; if visible, destruction fire is used. Light crew-served weapons are best attacked by neutralizing the crew.
AP and HEAT ammo can be effective against vehicles, and HE on a direct hit due to the minimal deck armor. AP and HEAT are nearly useless against other targets.
Artillery usually has great effect against armor only in assembly areas, when the vehicles are densely packed, or when a vehicle is immobile, when precision methods can be used. Air burst HE and richochet can be used to force vehicles to button up.
Artillery's aim against armor is to force them to button up and to eliminate tank-riders (who can basically be treated as if they were standing). Actual kills would be rare unless the target was immobile and precision fire could be used. 
Soft vehicles such as trucks, halftracks, jeeps in an assembly area can be attacked by unobserved fire for neutralization or precision fire for destruction. HE with quick fuse, or WP if combustible, is effective. On roads, precision fire first stops the column then destroys each vehicle in turn. This is best done in a place on the road with no easy exit, such as at a defile or culvert. Landing craft should be attacked with airburst or direct fire.
Personnel in halftracks are about 1/10 as vulnerable as troops in the open; trucks, about 1/4. As with armor, direct kills with artillery will be rare, but these vehicles will be immobilized about three times more often than tanks. 
Heavy bridges are very difficult to knock out. Heavy-caliber HE with concrete-piercing fuze is most effective. It's easier when the bridge axis runs along the line-of-fire, since the dispersion area is much longer along that axis, and a direct hit is required.
Wooden and pontoon bridges can be knocked out by almost any artillery.
Precision fire with large-caliber HE and delay fuze can create craters. It is most useful where not easily bypassed.
Quick fuze on HE shells in woods may cause detonation in the trees. This may decrease the effect if the shell goes off high in the trees, or it may increase the effect by acting in the same manner as an air burst.
VT fuze is useless in woods unless the angle of fire is very great, in which case most bursts occur at their normal height.
Woods have little effect on low-angle quick fuzed HE fire (other than making observation difficult), since the cover effect is offset by the detonating effect of the trees. High-angle fire, on the other hand, is about twice as effective as on open ground. Personnel in the edge of woods are in great danger from direct-fire HE, as almost any shot into the tree will act as an air burst. 
Smoke in woods will take about half as much ammo, since wind is reduced.
HE shells with long-delay fuzes (concrete-piercing) can be effective against fortifications, though it requires a direct hit. Close-range direct fire is best, with cover from smoke, covering fire, or darkness. Several direct hits are usually necessary.
Precision fire with pieces 155mm and larger are effective against heavy buildings; however, rubble is almost as useful in fighting as the buildings themselves. Light buildings can be destroyed with HE/quick fuze or WP. Quick fuzed shells striking light buildings containing infantry will usually be deadly to that infantry, since the blast is contained and highly on-target. Light buildings have many of the same effects as woods. Lower floors of heavy buildings and basements offer substantial protection and the building must basically be collapsed to get to the infantry within.