General:
* Before
* Previously
* Formerly
* Prior
* Ago
* Sooner
* In the past
* Previously mentioned
* Aforementioned
More specific:
* For time: Beforehand, ahead of time, in advance, antecedently
* For order: Prior to, preceding, antecedent to, foregoing
* For comparison: Preceding, foregoing, antecedent
* For emphasis: Long ago, ages ago, a while back, way back when
The best synonym will depend on the specific context of your sentence.