There are a few preserved trenches in north eastern France. My favourite is Vimy Ridge, a Canadian system very neatly preserved and staffed by very friendly Canadians.

On the whole though, most of the trenches are long gone. Some are marked with white stone running through fields, especially on the Somme. Alongside these, small graveyards can be found all over the place where casualty clearing stations were, just a few hundred metres from the front. As the trenches moved on forward, so did these stations. When men died there, they were buried straight away, which is why there are so many of these in the area.

EDIT: spelling