You could attempt to adopt a strategy of always following your commitments. From your current perspective this is useful but once you have learned your strategy has failed, what's to prevent you from just disregarding the strategy?

Disregarding it once will convince yourself and others that you will disregard it in the future, and remove your ability to make other precommitments.

The nuclear war example is more complicated, because presumably having a nuclear war will be the last thing you ever do. I would credit it to evolved instincts. Evolution "knows" that precommitments are important, so it gives us the desire to follow them even when it is not immediately rational to do so - for example, a lust for revenge that ought to be sufficient to make us retaliate in nuclear war, or a concept of "honor" that does the same.

I'm not saying commitments aren't useful, I'm just not sure how you can make them. How do you prevent your future self from reasoning their way out of them?

Our brain has several mechanisms by which we can make commitments: Honor. Pride. Duty. Guilt. You can use any of those emotional mechanisms to the service of enforcing commitments.

Because CDT isn't rational. You don't always have to act only for the sake of things that you can cause. If you're a transparent agent then you sometimes have to become the kind of agent that will carry out a precommitment. If that commitment fails then the rational thing to do is to carry out your threat.

EDIT: No-one else in the thread appears to understand that you don't need to have an additional reason (like a third party agreement) in order to carry out your threat.

I'm not saying commitments aren't useful, I'm just not sure how you can make them. How do you prevent your future self from reasoning their way out of them?

People don't normally do things because that would be the rational thing to do. They do things because they believe to be the kind of person who does such things. Usually you would have to train to overcome that bias, but in this case you can make it work in your favor. So here is the three step program to learn to precommit:

1. convince yourself rationally that being able to precommit has a great expected utility and that hacking yourself to be able to precommit is a good thing.
2. Make lots of small easy to follow precommitments like precommiting what to have for lunch, etc. But always be sure to doublecheck that you will actually be able to do it and won't be inconvenienced by it. When it is time to follow through on them remember you're not doing it for the precommitments but for a "higher good".
3. When you have followed on a precommitment tell yourself aloud: "I am the kind of person who always follows through on precommitments."

That should make precommitments second nature to you.

The most obvious solution is to coerce your future self, by creating a future downside of not following through that is worse than the future downside of following through. Nuclear deterrence is a tough one, but In principle this is no different from coercing someone else. (I guess one could ask if it's any more ethical, at that...)

See a lawyer and notary and sign a contract. Be skeptical of precommitments when this isn't a realistic option.

It would seem to make it impossible to commit to blackmail when the action of blackmail has negative utility. How can you possibly convince your rational future self to carry out a commitment they know will not work?

You put the answer in the title. We are humans, not rational agents. We have built in mechanisms to handle this. Pride, embrace it. This actually becomes easier with experience. I've found that in times when I've tried to be a good little CDT agent and suppress my human instincts it has gone badly for me. My personal psychology doesn't react well to the suppression and I've actually been surprised how often failing to follow through with a threat (or what should be an implied threat) had more negative consequences than I anticipated. On this my instincts and my ethics are aligned.

Use a third party, preferably a binding legal contract or similar.

Ideally, your decision to follow that precommitment should be so strong that you don't really have a choice, retaliating is something you don't even think about but execute by default. With precommitments, you want to restrict your own decision-possibilities.

If I hadn't dissolved the question already, I'd probably have come up with something like "by making precommitments, you want to undermine your free will so that once that events (nuclear strike etc.) happened, you don't have a free choice anymore because your free will is nonexistent in that situation".