When students find the area of a parallelogram, make sure they use the length of one of the sides and the perpendicular distance between that side and the line parallel to it and not the length of the two sides. This is a common mistake that probably carries over from finding the area of a rectangle.
Make sure students properly label answers in square units, not units, when finding area.
Constantly reinforce what area means by stating things like, "The area of this figure is 15 square inches, which means I could take 15 square-inch tiles and completely cover this figure." Encourage students to verbalize their responses in the same manner.
On a large poster, draw a diagram of a triangle enclosed in a rectangle like the one below. Post it on the bulletin board or wall in the classroom. Remind students that the formula for the area of a triangle is A = 1/2(b x h) by writing it above the diagram. Do the same with a diagram of a parallelogram.
Providing students an opportunity to review these formulas and using them throughout the year will help them remember the formulas for next year.