I mostly agree with this Ezra Klein post that beyond the issue of the Clinton campaign and its tactics is the issue of Clinton the Senator, who worked only on small-bore legislation instead of leading high-profile fights that would have made her the candidate of change.
Still, tactics also play their role, and I disagree with the conventional wisdom that the campaign's mistake was in its failure to put sufficient resources and time into the post-February 5th contests, where Obama has picked up unstoppable momentum. This argument seems to believe the money and time spent on the later contests would have come out of thin air, when in fact it would have reduced her position in the earlier contests and give Obama more momentum and "hope" in the later fights.
You don't beat a phenomenon by outlasting it, your only chance is to snuff it in the bud. The Clinton campaign's mistake was in not diverting more resources to the pre-February 5th campaigns, especially Iowa. Had she gone all-out there and won, combined with her New Hampshire win, it could have been different. African-Americans only switched to Obama when he had proven himself viable, so in this scenario she might have kept South Carolina and the campaign would have been over.
It probably wouldn't have worked, and then everyone would say it was stupid not to plan for a long campaign and I'd have no effective response, but I think the odds are better than the odds Clinton now faces.
As for why I posted on February 7th that Obama would win when most of analyses I read seemed to think Clinton had done well - Obama fought her to a draw when people knew her the best and him the least. I thought people tended to like Obama more as they heard him more and got used to him as presidential material. The average voters in later contests have more time to get exposed to him, so I felt he would keep doing better. I'm not sure that this is the right reason, but it does seem like the right prediction.