1. A moisturizer.
I used to believe that since I had oily skin, moisturizing was the last thing I needed to do. I was wrong! You're skin's oil is not the same as "moisture." It's not hydration. You can't water a plant with oil, right? So you can't expect your skin to be hydrated by it's own oil, either. Hydration (moisture) means water, not oil, and all skin benefits from added hydration, since the skin's internal water content tends to evaporate. So in addition to drinking 64oz of water a day, choose a fragrance free, "oil-free" moisturizer to add extra hydration to your skin.
2. A primer.
A good primer is the key to keeping your makeup on your face especially in hot and humid weather. Even if your sweating, primers will keep your makeup in place.
3. Finishing Powder.
4. A finishing spray.
5. Blotting sheets.
Honorable Mention Milk of Magnesia