Unlike many slaves, the Helots were Greeks who outnumbered the Spartans who held mastery over them. They several times rebelled against the Spartan rulers, at last successfully.
Yet it seems that Helots in large numbers accompanied the Spartans' into battle. How was order maintained? Why didn't they turn on the Spartans in battle? I would also be interested in any references describing the Helots and the techniques Sparta used to control them.
Normally the Spartans were very much concerned about helots uprisings, and there were indeed several large recorded ones. On the other hand, when needed (when pressed really hard), they would draft helots to their army, and usually they granted them freedom for this. Several such instances are described by Thucydides and Xenophon. There is indeed no recorded case that I know when those liberated and drafted to the army turned their weapons against the Spartans. But why should they? To achieve what? They were already liberated. You may argue that the purpose would be to liberate other helots, but this is apparently not the way they thought.
Of course these liberated helots did not become real Spartans with full rights, but they became personally free, probably similar to ''perioikoi'' (which constituted a large part of the population). But this was similar to the situation in other city states: not all inhabitants had full citizen rights.
By the way, Athens also drafted slaves to their navy, when pressed hard.
First of all, helots were "light infantries" only, at least until Brasidas' campaign. So spartan hoplits could defeat them in a direct clash: until Iphikrates light infantries had no tactic against heavy phalanx, no matter if helots outnumbered spartans as 7:1.
But why they never tried to betray spartans in the battle seems really strange. Of course, some helots may hope to deserve a freedom, but there couldn't be too many of them.
It seems that the most popular guess is that the helots of Laconia had some privileges over the helots of Messenia (kind of "home slaves" vs. "rustic slaves"), and only privileged helots could be soldiers in the army of Sparta.
Lechaeum, 390 BCE In fact, I've mentioned Iphikrates in my answer. But given that his success was so much unexpected by his contemporaries, it wasn't really an easy thing to do. Considering "simply running away", anyone could throw out his shield and run away successfully if there's no enemy cavalry around. But that's not how the battles are won.
I would also be interested in any references describing the Helots and the techniques Sparta used to control them.
On the techniques used by Spartans to keep the helots in check you may want to read about the so called “ceremony of the helots” in Thucydides, IV,80. Thucydides describes how at one time the Spartan authorities got rid of around 2,000 helots who had fought on the side of the Spartans and were expecting to be liberated as a reward. The Spartan authorities called forth all those among the helots who regarded themselves worthy of reward for having provided good service to the Spartans in battle. From those who turned up, the Spartans picked 2,000, whom they dressed in ceremonial clothing and paraded around the religious cites of the city, never to be seen again.
Thucydides does not date the incident. He mentions it as an example at the beginning of his account of Brasidas’ campaign of 424 BC, in order to support the claim that the Spartans were constantly in need of taking precaution against a helot uprising. Due to the context in which Thucydides makes his digression on the 2,000 helots, it has been argued that this incident took place not very long before the campaign of Brasidas {Jordan (1990)}. However, there is good reason to think that this is a much earlier incident that could have taken place sometime after the battle of Plataea. You may want to check also this question if you are more interested in this topic.
