How did the Romans (for example) date the year before Christ died?

They counted the year in terms of Emperors’ reigns.
For example the year Hadrian IV, Augustus VII; that kind of thing.

In England, that practice carried on well into christian times. They would say something like “The fourth year of Ethelred”.

Roman historians marked time from the founding of the city of Rome, which is conventionally held to be 753 BC; average Romans marked time by who the consuls were in year they cared about. Marking time by the years of the emperors’ reigns obviously didn’t work in the days of the republic (some 700 years), and was more a Byzantine practice than a Roman one.

http://en.wikipedia.org/wiki/Ab_urbe_condita

The romans counted years from the traditional date they believed Romulus founded Rome, 753 B.C.E.