In common spoken English, subjunctive is used when you are talking about a hypothetical future, or a wish. There are other cases, too, but these are most common.

Here are examples:
-- Are you going to the party tomorrow? No, but I wish I were!
-- If I were a rich man, I would build a big tall house with rooms by the dozen.
-- If I were elected president, I would promise to ...

Here is a wikipedia article all about it with lots of other examples: http://en.wikipedia.org/wiki/Subjunctive