gistfile1.txt

trigger mytrigger on object(before update){
  for(Object current : Trigger.new){
    Object olObject = Trigger.oldMap.get(current.Id);
    if(olObject.MyField__c != current.MyField__c && otherConditionsGoHere){
      //Option1
      olObject.MyField__c.addError('Cannot do that, Hal..');
      //Option2
      current.MyField__c = olObject.MyField__c;
    }
  }
}