Take care of your clothes. Your clothes should be in good shape(stains and holes are a BIG turnoff), and not wrinkly. If you have to, iron your pants or shirt.
Trying to go out in a button-down while attempting to hide a missing button, whether at the bottom of the shirt or on the cuffs, is a classic mistake.
Girls are very keen on minuscule discrepancies like these and will be the first ones to notice.