After Effects CS6 : Template Compositions
There are a few different ways you can do this depending on your scenario.  Normally, for things like lower thirds, I make one composition in which I enter just the lower third text and plate.  I then use that composition as an asset in another composition to do the actual animations.  I can then simply turn on or off any particular text I need to in the static composition and render the final composition to get the needed output.

Similarly, for a video at the beginning of a clip, you can have a wrapper composition that has the lead-in and then you can replace the contents of the composition that is shown after the lead-in plays.

Alternately, you could probably do a fair bit of it using expressions too, but I've yet to have any tasks that were complex enough to have bothering to setup something with expressions.