Rub the stain with liquid detergent, or a pad soaked in methylated spirits, and rinse thoroughly. If the stain persists, try a commercial stain remover on it, a mild solution of hydrogen peroxide, or - if safe for the fabric - bleach.
If you're in a hurry, dab the stain with a cotton bud soaked in water and a little methylated spirits. Be patient, it may take a few minutes for the stain to fade.
Cover a permanent stain with an applique or a patch.
