Will always depend on which race you've chosen and which you're facing, but the number one thing I can recommend is to not be afraid to use your workers en masse to break an enemy's rush- the most common tactic in the game, particularly for Zerg. If you just make your workers run away you've probably lost anyway from income difference, so you may as well go down fighting. I once saw a group of 5 rushing marines beaten by well-managed probes, losing only half their numbers. The Protoss player rebuilt, rushed with Zealots and went on to win the match.
Find a choke point (usually a ramp) and fortify it with buildings if you're Terran. This works for Protoss too but not as well, though I consider their Photon Cannon to be the best fixed defense building in the game- build lots of them at your main base's field since their other Detector unit is usually too limited in usefulness to waste resources on. In general your strongest defense should be based around your mineral patches and geysers, as another extremely common tactic is to drop/warp lethal units such as Reavers or Hydralisks onto an unprotected field. Death and frustration ensues.
Find a choke point (usually a ramp) and fortify it with buildings if you're Terran. This works for Protoss too but not as well, though I consider their Photon Cannon to be the best fixed defense building in the game- build lots of them at your main base's field since their other Detector unit is usually too limited in usefulness to waste resources on. In general your strongest defense should be based around your mineral patches and geysers, as another extremely common tactic is to drop/warp lethal units such as Reavers or Hydralisks onto an unprotected field. Death and frustration ensues.
