I just watched some heartbreaking footage of soldiers going 'over the top' of the trenches and storming into no-mans land, then getting mown down. Surely some kind of shield, even a rudimentary wooden one, would provide a degree of protection from incoming bullets?
I'm not really sure how to approach this question because it's uhm, a bit weird. I think you really underestimate the penetrating power of the rifles used in WWI and especially WWII. The type of shield that would be necessary to not only stop high powered rifle rounds from the average soldier but fully automatic fire from various models of machine guns would have to be so incredibly thick and heavy (and metal) that it's practically useless. It would also require so much metal to make this for every soldier you wouldn't have any metal left for anything else.
A "rudimentary wooden shield" would do absolutely nothing to stop a machine gun emplacement from tearing through a person or a rifle round from going through it. The Germans issued heavier bullets for machine guns and those would easily penetrate 1-2 feet of wood and up to 3 if we're talking about closer ranges (<100 meters).
Even if personal shields were not cumbersome as hell and incredibly expensive to make it does little to stop the fact that over half of the deaths in the war were from artillery.
Weight.
Steel weighs around 8 metric tons per cubic metre. A 12mm thick steel shield sufficient for a man to hide behind (roughly 6x2 feet) would weigh over 100kg, and this would only barely be bullet proof.
A soldier so equipped would have to be incredibly strong to simply hold the shield up, let alone conduct tactical manoeuvres and fire his rifle at the enemy.
And as /u/elos says, such a shield would be useless against the main causes of infantry casualties.
Related discussion thread from a while back. http://www.reddit.com/r/AskHistorians/comments/1mm0d8/why_didnt_the_allies_use_ballistic_shields_when/