The idea of using your mind to change your brain in order to overcome negativity bias (Hardwiring Happiness: the TED talk) fascinates me.
The general technique is this: Either in the moment of a good experience or by recalling a good experience. Then, hold it in your mind for 10-20 seconds, letting yourself sink into it, relax into it. Allow it to be absorbed. And then if you want to, you can also use this positive memory to merge into/work with a negative memory, which will gradually shift the negative toward positive.
I find this fascinating because it turns out that I have been doing this without knowing that was what I was doing, for a long time. I learned probably 20 years ago that focusing intentionally on the positive experiences set me up to look for more positive experiences. I had read a study at some point that showed that people who were looking for positive experiences encountered them 25% more often than people who were expecting negative experiences. Most of that was simply because positive people were observing and assessing the opportunities for potential positives, first, and so they saw them instead of missing the subtle ones.
So, I decided to always be looking for the positives. In order to remind myself to do so, I'd fish through my day mentally and find as many positive experiences as I could, and turn them over in my mind, enjoy them, even if they were blips or small moments. Eventually, I started taking negative moments and looking for positives that were tied to them in some way, like someone helping me pick up the broken bits of a cherished mug I broke, and being sympathetic about it. It wasn't 'oh, I broke my favorite mug, that sucked (and background, yeah, okay someone helped me with that, but it still sucked!)', but instead I shifted intentionally to 'I was having a hard time and someone was really kind. I'm bummed about my mug, but hey, people stopped and helped, and that mattered!'
This is such an ingrained habit now that when I'm telling people about the auto accident where I broke my sternum - and was pretty high risk for dying right then, and spent a lot of time in pain both then and later - what comes out is more 'my boss is awesome, (in the time I was most likely to die without warning and had to just live with the fear because there was not a blessed thing I could do about it) he stayed on the phone with me so someone who cared about me would be there with me if the worst happened', or 'After the first three seconds, I knew my worst risk was past, AWESOME! (even though I hadn't made it out of the car yet and was still overwhelmed and very rattled)'. or 'wow, the bystanders (who helped me get out of my still smoking wreck of a car that I'd just bought a couple months ago) were incredibly helpful!' ... etc. I know I used to approach everything from a worst case catastrophe perspective, but now I can't imagine approaching life in any other way. Even though I don't at all think 'everything happens for a reason', I am always mining the things that happen for things that are useful, kind, good, reassuring, loving, creative, etc.