What I like to do is simply write the essay without worrying about the linking ("to be" verbs), then simply go back and change the sentences so that they don't have any. Take this example:
Sally is at the store. She is looking in a freezer to buy ice cream.
Simple, yes. To change it over to action verbs, I would say something like:
Sally shops at the store every Saturday at noon because she always runs out of ice cream.
Although this changes the meaning of the sentence slightly, it only does so because the change includes more description of Sally's reasons for going to the store and places Sally as the subject of the action; in other words, it's not just an objective description of her.