Similar to the issue of passive voice, overusing a form of "to be" (am, is, are, was, were) is one of the easiest mistakes to fall into when writing a paper. After all, "to be" seems to seem applicable for any situation. What this overlooks is that there are a variety of better, more interesting verbs that might be used instead of the static verb "to be". Reread some of your old writing with this in mind and you might suddenly notice the repetitiveness of the use of 'to be' verbs. When editing a paper, strive to notice any and all times a form of 'to be' is used and think of ways to re-write the sentence with a more interesting verb. Thesauruses are your friends. Compare these sentences:
The fireworks were bright and noisy. They were lighting up the night sky.
The fireworks exploded and boomed, lighting up the night sky.
Just like that, your writing becomes simpler, cleaner, less repetitive and more interesting.