There are some common and annoying mistakes which may significantly harm your grade. However, you can avoid those grade lowering mistakes by completing the following checklist:
Plan B: Use Plan B if you have only a few, larger similarities or differences. After your introduction, in the next paragraph discuss one similarity or difference in BOTH works or characters, and then move on in the next paragraph to the second similarity or difference in both, then the third, and so forth, until you're done. If you are doing both similarities and differences, juggle them on scrap paper so that in each part you put the less important first ("X and Y are both alike in their social positions . ."), followed by the more important ("but X is much more aware of the dangers of his position than is Y"). In this format, the comparing or contrasting goes on in EACH of the middle parts.