Clichés - Banish them completely. Always search your mind for a more effective and unusual way of expressing your meaning.
Pomposity - Never try to be over-clever with language. Never talk down to the reader. Use your thesaurus by all means but not to show off with long words.
Dialogue - Sift out any words or phrases that don’t suit the character who is speaking.
Journalese - Beware of sounding like a journalist trying to fit everything into one sentence.
Dialect - Use with caution. Confine it to one or two key-characters and tone it down.
Slang - Check that any particular word or phrase was used in the period of which you write. Modern slang soon goes out of fashion. Use with care.
Foreign words - Restrict to a few essential ones to give a flavour, like cherries in a cake.
Jargon - If you are writing about a specialised subject, make sure the meaning of any technical words or phrases are made clear.
Clarity - Make sure that this applies to all your writing.
Check spelling, grammar, punctuation, split infinitives, repetitions, clumsy sentences etc. Cut out purple prose, useless adjectives and adverbs and all superfluous material which adds nothing to the story while trying to keep its vitality and freshness.
To help you do this, keep in mind your reader who you should be encouraging to use his/her imagination. Understatement is a powerful tool.
It is your responsibility to edit - not the editors. Send in as clean a copy as you can.