In philosophy, you spend years learning how to attack arguments. If you keep doing philosophy, you'll attack others and they'll attack you in what feels like a kind of constant epistemic trial by fire. It's not always fun, but it does seem to make people better at arguing.
Sometimes people ask how they can hone these skills. The least useful answer to this is some variant of "sorry, you just have be good at it". The degree to which argumentative skill is an innate talent is unclear. Even if most of those who end up in fields like philosophy are often innately good at it, this could just be an example of an unfortunate selection spiral in which only those who are innately good at the thing pursue it, and therefore only those who never really needed to be taught the thing end up teaching it.
A slightly more useful answer involves recommending texts on critical thinking, classes in formal logic, and so on. But this isn't how most people become good at arguing. I haven't ever taken a critical thinking class, and I didn't learn formal logic until after I had already developed a lot of the skills that I'm talking about here. So what's going on?
I once heard that sharks generally don't bite people because they want to eat them. They bite people because they reflexively bite at anything that looks kind of like a fish (which can include humans) and because biting us is their way of trying to figure out what we are.
Like the intellectual equivalent of sharks, people who are very good at arguing seem to have a habit of reflexively attacking most claims and arguments that come their way. For example, they might see "up to 40% off" and get annoyed by the fact that the claim tells you nothing except that the store definitely won't give you more than 40% off, which can be claimed by a store offering 0% off. Attacking a claim is their default response, even if the claim is fairly trivial.
For me, this reflex is often at its strongest when I'm confused by something. If someone puts forward a claim that doesn't make sense to me, I do the intellectual equivalent of biting it to figure out what it is (i.e. I try to tear it apart). This strategy can be pretty effective, since people will often put effort into clarifying what they mean when their views are challenged.
So an effective way to improve your argumentative skills and become a clearer thinker may be to become more curious about the world and, at the same time, more aggressive towards it. You investigate more things, but your reflexive method of investigation is somewhat bitey. We can call this the "curious shark" approach. This strikes me as similar to what a lot of philosophy programs actually do in practice. They throw argument after argument at you and force you to come up with counterargument after counterargument. In order to get better at both defending and attacking, you'll probably try to learn some logic or probability theory, but it's the unrelenting practice that forces you to find better strategies over time. (Alan Hájek has helpfully distilled some of the strategies that many philosophers converge on.)
I think this partly explains why philosophers often end up defending pretty weird views. The discipline of philosophy is obsessed with argumentative prowess. Since it's not all that hard to argue for something that most people find plausible, those arguments are not very impressive. But if you manage to argue that all possible worlds are real and meet the inevitable argumentative onslaught that follows, that's pretty damn impressive. Arguing for an implausible conclusion is like tying your hands behind your back before entering a tank full of sharks. You're definitely going to get attacked, but everyone will be all the more impressed if you come out successful.
One problem with the curious shark approach is that, from the point of view of anything they bite, sharks are assholes. That's not bad for the shark because they don't particularly want to make friends with the things they're biting. But people do want to make friends with those around them (or at least not lose friends they have), and constantly tearing down their arguments isn't exactly the best way to do that.
A related problem with the approach is that most ideas have to start out life as vulnerable little fish before they can grow into something more robust (see this post). If you create an environment where people have to defend their ideas from hungry sharks from day one, people will learn to either hide their ideas or stop coming up with ideas in domains they're not already extremely well-versed in.
This was true of my philosophy grad program. It was a competitive environment, which was good for honing your ideas once you'd been working on them for a while. But it felt like no one really wanted to express nascent ideas. You knew that if you put forward an idea it would be attacked ruthlessly. So it made more sense to hole away and do the work yourself, and to only show your ideas when they had grown robust enough to withstand the attack. This is unfortunate because early discussions of ideas can be extremely helpful, and is presumably how you get the most value from having other grad students around.
I've also experienced the other extreme. I once went to a conference that was trying to move away from the traditional aggression seen in philosophy conferences and embrace a more supportive atmosphere. I thought I saw a problem in a paper and stated it honestly in the Q&A. I felt like my problem was never really fully addressed but most of the remaining comments were things like "here's an interesting domain where your analysis might apply" or "have you read so-and-so's related work? I think you'd like it." At the time I felt like I'd breached a social norm by pointing out a problem with the paper so bluntly, but I also felt like I was doing a bigger favor to the author than any of the more supportive commenters were because the paper would be strengthened the most by fixing problems like the one I was pointing to.
So what are we supposed to do here? If we're too aggressive with ideas we can kill promising but unrefined ideas when they're most vulnerable, but if you coddle ideas you can fail to strengthen them early on and set them on the right path (or, worse, let someone work on an idea for a long time that really should have been abandoned much sooner).
Some people try to get around this dilemma by distinguishing between aggressive content and an aggressive tone. The thought is that if we deliver our biting criticism with a kinder tone, we can avoid the chilling effect that comes with biting criticism. It's true that an aggressive tone can make an intellectual attack feel even more stressful, and perhaps an aggressive tone should never be necessary. But I don't think a friendlier tone would fully eliminate the chilling effect or the "you're an asshole" effect. It's a little bit like moving from a barroom-brawl to a well-regulated boxing match: rules might help, but getting punched in the face is still going to hurt.
Here's the only thing I've found that helps: I point out problems with ideas at every stage of development, but I try my hardest to solve any problem that I identify. Even if I don't succeed in getting over my own objection, I make an effort. If you show that the goal of your attack isn't to merely destroy the other person's idea and declare a personal victory, but to jointly get at the truth and build on whatever part of the idea seems promising, the attacks you level are more likely to have the effect of strengthening rather than killing a promising but unrefined idea. And if the idea does die (as some ideas will), it's more likely to do so because you've both tried to make it work and jointly concluded that it won't, which ideally doesn't discourage the other person from voicing similar promising but unrefined ideas in the future.
So if you want to become a sharper thinker, the adversarial training you get from habitually attacking ideas and welcoming attacks from others seems pretty effective. But I think you can do this while minimizing the chilling effect and the "you're an asshole" effect by treating it as your job to try to counter your own attacks to the best of your ability. I'm not sure if this is the best solution to this problem, but it's the best one I've come up with so far.