A comment on a student paper that you may find perhaps useful:
If you find yourself naturally pausing, there is likely a comma or semicolon needed. (Read that last sentence aloud right now; do you hear a pause?) Could the parts after and before the pause be complete sentences? If so, it is a compound sentence; put a semicolon at the pause. If either side of the pause could not be a complete sentence, this is a complex sentence; you need a comma. If you find that you have multiple pauses, look carefully at how long your sentence is. You may be trying to put too many ideas in a single sentence.
