Never quote. If you paraphrase you show that you understand what the author is talking about.
Consider both sides of an argument and reference.
Consider several points and link them together to aid the flow of the body.
Back up any ideas you have with sources (kind of obviously really, but easy to forget. You can't come up with your own theories at undergraduate level).
A strong introduction that says what you're going to talk about, and a conclusion that wraps everything up but avoid saying the exact same thing over again in condensed form. The conclusion should also refer back to the introduction in some way.
And RTFQ. Too many people in my group at uni do poorly at essays because they don't answer the question. They might write a great essay but it means nothing if it isn't what is asked from you.