Try aloe vera. Apply aloe vera gel (available in many drugstores as well as health-food stores) to your face to absorb oil and clear out pores. Dab the gel onto your face two to three times a day (especially after washing), then let it dry. The gel will feel more refreshing if it's cool, so keep it in the refrigerator.
Wipe with astringents. Wiping the oily parts of the face with rubbing alcohol or a combination of alcohol and acetone (a mixture found in products such as Seba-Nil Liquid Cleanser) can help degrease your skin just as well as more expensive, perfumey astringents. Many drugstores even sell premoistened, individually wrapped alcohol wipes that you can keep in your purse for quick touchups throughout the day.
Carry tissues. Even if you don't have an astringent with you, paper facial tissues can help soak up excess oils in a pinch. You can also purchase special oil-absorbing tissues at the cosmetics counter that are very effective in removing excess oil between cleansings.
Chill out with cold water rinses. If you don't want to apply chemicals to your skin, simply splashing your face with cold water and blotting it dry a couple of times a day can help remove some excess oil.