Here are a couple of things I havent seen mentioned:
1) Bike selection. Most bikes you look at are cross country bikes, they are great climbers but more sketchy downhill. They are characterized by a head angle of 70-71 degrees (called steeper head angle). This means the front wheel is closer to the main triangle. This is good for climbing and faster handling. The next level of bike is a trail bike (68-69 degree head angle), the front wheel is moved forward like a chopper (called slacker head angle). Moving the front wheel forward means a lot more stability going down hills, but a little more difficulty climbing. I chose a trail bike (tallboy LT) because I wanted to feel safer going down hills and I was willing to take the hit on climbing. I can bomb down technical gnar that people who have been mtb many years more than me are hesitant to take. It isnt me, its the bike. With a slacker HA it is just much harder to endo
Examples of XC bikes would be the specialized epic, where the stumpjumper FSR is more of a trail bike. The giant anthem is a XC bike and the trance is more of a trail bike.
2) I read a bunch of people mention manual/wheelie drops. People implied that this means you have to pull up on the bars. A much easier (and I think more effective) way of hitting drops is to "huck" the drop. To do this you crouch into attack position then actually push your bars forward as you hit the drop. It is totally counterintuitive so i would say virtually impossible to discover on your own. There are plenty of youtube videos describing this. The advantage to this method is that it is much easier to get the timing right and you stay much more in control because you are not shifting your weight around. You will land with the rear wheel first or both wheels down at the same time. You can learn to huck drops in about 30 minutes of practice. I was doing 3-5 ft drops after just 30-40 practice hucks. This technique completely revolutionized my riding. It is hard to practice off curbs because they are a little short. But if you have rollers or small ledges on your trail instead of preloading and jumping, crouch and push your bars forward as your crest the roller.
here is an example of a video which shows the technique
http://www.youtube.com/watch?v=rBnarkUPxuo
3) practice manuals, wheelies and bunnyhops on a 26" bike using platforms with pins and good flats like fivetens. It is much easier to learn them on a 26 than on a 29er because of the shorter wheelbase/chainstay length. For wheelies, pedaling as you lift the front up a grassy hill will pop your front up. Think about how light the front end gets when you try to climb a hill. Use that to your advantage to learn pop wheelies.
4) When climbing, the nose of your seat should be in your a** crack. You can slide your butt backwards if you lose rear traction. A lot of times I shift my weight back as I start to crank the pedal to keep the rear from slipping, then shift forward as I finish the pedal stroke to keep the front end from hopping. You basically are sliding forward and backward in time with the pedal stroke. It may just be a matter of a few inches.
5) When climbing use a higher gear than granny. You may think granny is easier, but a lot of times you put down too much change in power and so the wheel slips. If you practice riding in a higher gear you will naturally hit the hills faster, but will put the power down more evenly as you climb the hill.
6) to build strength ride trails in a higher gear than you are used to. You will fail to clean things you used to, but you will build strength. On one trail there were 4 or 5 places that I would switch to 1-2 but now I can ride the entire trail in 2-2 or above. Experienced riders ride the entire trail in 2-4. Another benefit is that at the same cadence you will go faster.