If you use a prologue, make it as good as the beginning of your first chapter. Don't wimp out on either one, and don't cripple yourself by starting out so large the only way to go is down. Ensure the who, what, where, when and why are included right off the bat. Double check your grammar, punctuation and spelling. Don't think if the story is good, the mechanics won't matter.
Judges can be pickier than editors.
Stay focused on your story. Include only those elements that advance the plot, reveal character or provide necessary description. Long narrative passages can bog your story down. One of the most common mistakes new writers make is dumping blocks of unnecessary back story into the first chapter. If not, weave it into the story later.