Purdue OWL's "Avoid Common Pitfalls." This page covers many elements that can make sentences harder to read, issues you'll want to focus on while editing.
University of North Carolina at Chapel Hill Writing Center's "Writing Concisely"
"A Demonstration of Richard Lanham's Paramedic Method." The paramedic method was developed by Richard Lanham. It provides a systematic way to look for and recast elements that make sentences less concise. The link leads to a video I created on this model.