The most obvious way that the Allies prepared to invade France was by bringing huge amounts of war supplies from the United States to Britain.  The materiel was stockpiled there in preparation for the invasion.

The Allies also started to do more to help the Resistance in France.  The idea was that the Resistance would help the invasion by doing various acts of sabotage that would make it harder for the Germans to respond to the landings.

Finally, the Allies put on a huge deception to convince Hitler that the invasion was going to come near Calais instead of in Normandy.