I agree and disagree with several of these points in a way.
1. Lose as much weight as you can off of your body. I don't say bike because losing un-needed body mass not only helps w/kg, but your ability to climb standing, which for me is helpful. By losing weight and training my weaknesses I went from a 3.88 w/kg 1 hour power to almost 4.7 since May. Combined with learning how to climb better myself, I have taken almost 12 minutes off of a UCI Cat 2 climb (50:35 when I first did it, now down to 39 something)
2. Learn to climb in many different positions and at different cadences. I made the mistake of doing what Carmichael suggested and climbing at 90-95 seated all last season. I didn't really get better at climbing. If you watch the pros and the KOM competitors, they change positions a lot. Get comfortable riding on the hoods, the tops, the drops on shallower climbs and likewise at different cadences. You can't use the same position and cadence with every single climb, it just doesn't work. My local climb that I do is a UCI Cat 2 climb and I can tell you that you couldn't do the same thing up the whole mountain. The tougher double digit portions require a lower cadence/more strength and I use a more hunched over position to use more of my glutes and hamstrings or ride out of the saddle. The easier sections work better with a higher cadence and in the hoods to help delay or offset fatigue. The switchbacks require the ability to stand out of the saddle to really preserve speed and accelerate. The general ability to stand helps me to stretch out, change positions, and use different muscles. Also, if you only train seated or in one position, then what happens when someone attacks in a race? Very few people could just sit there and spin and be fine. I firmly believe that you have to train everything on a climb so that you are ready for every scenario and then you can more easily figure out what to do when. Climbing fast requires you to maintain a decent pace, which might require you to employ different techniques to keep the same speed and power output. You may never be Andy Schleck, but I've noticed that most of my climbing mistakes come from an inability to keep my pace and output constant because I over or under gear a specific section and end up in an uncomfortable situation.
3. Climb a lot, but not always hard. I know people have said 2 days a week max and I don't know where they got that from. You won't adapt any better or worse unless you only do 1 type of riding at 1 intensity level all the time in the same position and at the same cadence. I climb hard once or twice per week, but some weeks I might climb 5 total days. Don't be afraid to find a mellower climb and spend time at endurance paces or lower exertion levels. Hell, some days I might climb 5,000 feet, but at an endurance pace and others I might climb only 1,200, but in harder intervals.
4. Train for what you suck at, then what you will do most. This is just my opinion, but its worked for me. If and when I move back to the U.S. it is unlikely that I will ever be riding climbs like the ones I have been on in the Alps. I train on them, however, because I feel that if I can get better at climbing for an hour straight or on harder climbs, then 700 feet of climbing in a lap of a road race won't be so tough mentally. I used to race a lot of crits and road races with shorter climbs, so I trained for that. I did lots of 3 to 5 minute intervals, LT intervals, etc, but never placed better. Then I just started to focusing on climbing duration and I improved a lot. My limiter was not the hill type, but overall endurance. My LT was decent, but in some races you're going to have sections at that level longer than your intervals, possibly harder than your intervals, and without as much rest as your intervals. My improving my aerobic endurance I gained 10% on my threshold power and don't have as much of a problem recovering between climbs, which was my limiter before. I could hang with anyone on the first few go rounds, but after that I was spent. That now frees me up to focus more on the specifics of the races I will do since I can repeat the necessary efforts over and over and over in race conditions, not just a few times with carefully spaced recovery intervals like in training.
I know people should focus on specific training, but the people posting (me included) are generally amateurs doing this outside of their 9 to 5. We're not scouting out Tour routes and planning our season around a single stage, we're trying to become better racers overall. I think a newer rider should focus on getting better overall in terms of aerobic endurance, raising their threshold, lowering their weight, racing more often, riding more, eating better, etc. before they think of structuring all of their riding around 1 climb in 1 race and so forth. Also, you can train that same grade all you want, but it will never go the same way twice. You might be in the bunch, on a break, off the back chasing; the scenario will never be identical to what you are doing in training and you can get close, but why not use the time and effort to build up your overall qualities as a cyclist? I saw quite a few people buy PM's last season and train this and that for a few circuit races and they still didn't get any better even though they trained every imaginable quality. The winners rode and improved their whole range of abilities and became smarter/better racers.
With that said you can increase your power output and never be a climb. Your physiology dictates your power output graph and although you can train to help your weaknesses, you will not be able to re-write your genes. Many pros have incredibly high thresholds and similar power/weight ratios, but excel at different types and styles of climbing. I will always have a downward sloping plot, but if I want to use my strengths I need to get to the end of a race, which means getting over the climbs. Also, lots of races have KOM and intermediate sprints, so learning to climb can help me with those as well since they're usually shorter and earlier on.