Ideally, the first thing to check is how good the fit is, or in other words whether tolerances are causing or contributing to the problem despite the nominal sizes matching. That way you have a better sense of what you're up against. If the post has a lot of slop with the collar loose, it's possible nothing will make it work, although that's an outlier.
Now that carbon grip is an everyday item, there's no reason not to use it in slipping post situations regardless of material. So start by cleaning everything, putting a generous amount of that on the post/frame interface, and making sure all the mating surfaces of the collar and bolt are clean and greased. That's about as much as you can do without changing parts.
If that doesn't work and it's a bolt-down collar as opposed to QR, the first thing to look at is how beefy the collar is, if applicable. The thicker and heavier, the harder it pinches the post and the less it itself flexes as the bolt is torqued. Slippage happening due to tolerances on frames with collar type clamps can often be obliterated by just putting on a stiffer collar.
If it's a QR collar, there are some bad ones of those in the world, and they can get worn/mangled with repeated hard re-tightening, so replacing them can be necessary. Bolt down ones clamp harder in general too, so if you don't care about QR then that can make sense. (The average case scenario of a slipping post in shops is it's a junky QR collar, and the fix is replace it with a bolt down one and slather with carbon grip. Usually fixes everything.)
If it's an integral clamp, or none of the above fixes it, and it's not an undersize post, then some of the more advanced, shop-tool-requiring fixes can come into play, such as reaming the seattube or knurling the post. Sometimes the seattube opening needs some reworking too.