How can I reduce sebum buildup in pores?

Topical retinoids. The topical retinoids (such as retin-a) are usually effective in reducing sebum. All require a prescription. If that doesn't work, some dermatologists will prescribe very low doses of an oral retinoid (such as accutane).
Keep it clean. Keeping your face clean and not oily is the key. http://www.webmd.com/beauty/skin-glow-13/minimize-pores.