Starcraft 2: how to hold an expansion w/terran forces?

I've been trying some skirmishes vs. the AI and it seems like whenever I take an open expansion (an expansion not adjacent to one's base) I always end up losing it, all units and all structures. What does it take to hold an expansion?
I'm not playing all infantry or all air, as a matter of fact I never really get any air units built at all.