Via Twitter I've had a lots of good advice from writers who also judge writing competitions. Here it is: digest and avoid!
NICK DAWS – editor, freelance writer and tutor
Stories that clearly haven't been proofread and/or don't abide by the contest rules. A waste of everybody's time!
SALLY QUILFORD – writer and columnist for Writer’s Forum Magazine
SUE MOORCROFT – novelist and creative writing tutor
Also read (and learn from) Tania's comment on the other post about writing competitionsReturn envelopes that are too small or already stuck down! you have dozens of stories to return, inadequate envelopes seem a bigger and bigger thing!