What are the landmarks that separate the lobes and segments?

The hepatic veins lie between segments; the portal veins lie within segments. (The left portal vein is the exception to this rule.)
Therefore:
The posterior segment is separated from the anterior segment by the right hepatic vein cephalad in the liver. More caudally, the posterior and anterior segments are separated by a line that bisects the angle between the anterior and posterior right portal veins.
The right lobe is separated from the left lobe by the middle hepatic vein (cephalad) and the gallbladder (caudal).
The medial segment of the left lobe is separated from the lateral segment by the left hepatic vein (cephalad), the left portal vein (mid level), and the ligamentum teres (caudal).