How to keep yourself from crying?

Tomorrow I'm leaving houston to visit my best friend in st.louis for 3 weeks. I've done this every summer for 3 years and every time I go I start crying when I have to get on the plane. I'm realy close with my dad, so I don't know I guess I'm just sad to leave him. But it makes me feel so stupid. And sad. I'm usually not an emotional person at all, like I never cry. So does anyone have any tricks you use to stop youself from crying?