If u r zerg, you can put lurkers in ur gatherers. If ur terran, and ur fighting protoss or terran, u can put 6 ghosts in ur gatherers. Then, when u see them coming, USE THE TECHNIQUE LOCKDOWN!! A ghost can use lockdown 2 times. If you are protoss, u can have 1 or 2 arbiters in your gatherers. When some1 comes, use the technique stasis field. Then move corsairs or carriers or whatever to where they are and wait 4 stasis field to wear off.

CreepColony's Opinion: Lurkers will work very well against drops. Lockdown only works against mechanical units, and usually when someone drops, the units are usually organic. So it's the hardest to stop drops with Terran. I would say get a bunker or two near your command center and surround it with tanks and missile turrets. For protoss, stasis field will work well, but it arbiters and very high in the tech tree. I think dark templars and reavers would work well earlier in the game.