In 1940 or 1944? If the former, make the French more competent in fighting the Germans off. Soon, the momentum of the German assault would wear off, but the French and British, lacking mobile warfare tactics the way the Germans understood them, would not go for the grand strokes of a blitzkrieg. The war would soon bog down.
If the latter, it's what Eisenhower expected would happen. If you deploy a few more trained Panzer divisions to France in 1944, even Patton would have trouble moving as fast as he did IOTL (you'd have to supply those Panzer divisions, though, and the Eastern Front was getting more dangerous then...)