Let's look at the person in question a bit. What grave mistakes did he make in his long reign?
Few, I would say, and most of them came from flaws in his personality.
Denethor was arrogant when it came to felling decisions, and heeding the counsel of others. This is why he used the Palantír in first place -- his motives were only the sake of his people, but he mistrusted others, especially Gandalf, and so used the stone to gain the knowledge about the Enemy in a way he thought to be first hand. Of course, one could neither blame him for trying, nor for being misled by Sauron... or could one? On the other hand, it also led so far that he "despised lesser men, and one may be sure he did not distinguish between orcs and the allies of Mordor" (Letters).
He rather trusted Saruman. Another point against the Grey there. Faramir, whom he correctly perceived to be on the side of the latter, he did not trust very much either...
The way the issue of the Ring was handled by those who took the responsibility seemed to him like utter folly. And wasn't he right from his own point of view? Of course, it is this very point of view which may be modern, but is a flawed one in the cosmology of Middle-Earth. As Tolkien said somewhere in Letters, "Denethor was tainted with mere politics: hence his failure."
Denethor never was more than a politician, he never was more than a steward either. The charm, glory and intuition of a king he neither had nor could be demanded to have anyway. So that would be one mistake then - his aversion against Elessar and his promoters.
As for Denethor's suicide: Gandalf may call it a habit of the wild heathen kings, but wasn't it considered the duty of a defeated ruler in our very own antique to plunge himself into his blade? And he didn't leave his people without a leader, one might argue; after all, the northern usurper was at the doorstep already. That he wanted to take Faramir with him can be excused by his by then bad mental shape and his firm belief that the case of the West was lost.
If Denethor had egoistic motives behind, for example, admitting Mithrandir to the archives, this still doesn't counter the political and rational benefit of this very action - and if the steward would read the same scrolls after the wizard, this could have only been a disadvantage had the wizard had anything to hide from the mightiest leader of free men at that time.
Any speculations about ambigious plans or intentions of Denethor have to remain just that. So, his intelligent actions were not far-sighted enough for the great demands of the time. He was no king, but a steward,a politician. We can assume he was well aware of that -- maybe it boils down to the fact that the core of Denethor's failures, flaws and fall is that very deficit, and the way he lived - or should we say could not live? - with it. He became mistrustful, chauvinistic in his politics, and lost the view for the world outside the borders of his realms. Thorongil may well have played a part there, for even if Denethor did not recognize the heir of Isildur, what he clearly could see was what popularity Aragorn quickly gained.
Denethor's view became the simple paradigm that all changes are for the worse and would only contribute to the fall of Gondor, and himself with it. This of course lacks the necessary differentiation. For his own policy of conservation, the events of that time proved the worst enemy, though; Gondor was not powerful enough to ward of Mordor alone, and Denethor might have thought that himself not being king might have something to do with that.
Goals too high, forces too low... certainly a tragic figure, and I may add my personal opinion that Denethor is, on a modern literary scale, one of the most intriguing characters in the LOTR for just that.
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