As well as the experience modifiers already given, units can earn or lose experience by performing well or badly in particular scenarios. Details of these are given below.
Cleanse/Night Fight: Any unit which claims an uncontested table quarter at the end of the battle earns +50 experience points.
Rescue: +100 experience if the unit is in possession of the objective at the end of the battle.
Take and Hold: +100 experience if the unit claims the objective at the end of the battle.
Meat Grinder: In a one-off game, it is easier to think of 'recycling' units as part of the sustained assault. However, in reality the unit isn't instantly brought back to full strength and sent in to fight again, but is replaced by another unit from behind the front line. If you are playing using experience, a unit which is wiped out loses experience points as normal. It does not come back on to the table, but is replaced by another squad. It may be just a case of filling in a new unit roster card each time a unit is recycled, or you may want to devise a system of bringing on fresh troops up to the points value of the troops that are being replaced. As you can probably tell, a Meat Grinder is gruelling affair for the attackers and it may be that you'll not want to commit your most experienced troops to this near-suicidal battle!
Sabotage: +100 experience if the unit destroys the objective.
Breakout: +100 if the unit manages to leave the table off a short table edge.