The Great Wall of Terran

Terran has an awesome way of keeping would-be rushers out of their base - the wall. If you're in a map with a single entrance to your base, you can keep out an immense amount of non-ranged attackers (which means all Protoss and Zerg early forces) thus relieveing you fram rush countermeasures.

You start by building your first supply depot, preferable at supply limit 8-9 at the entrance. Once that is done and you have enough minerals (while constantly pumping out SCV's - economy takes precedency) build the barracks next to it, and seal it off (in most maps) with another supply depot.

This leaves your base closed off, so melee attackers will have to attack your buildings while you can shoot them with your marines.

As an extra bonus, if you build the second supply depot while the barracks is still underway, you can often close off your base, even from spying probes. This is a great way to keep the enemy in the dark!