I think it's important to define what accountability means to ourselves, and who it is for. After infidelity, many couples often set rules and conditions that are supposed to address the WS's lack of accountability and to provide a system that monitors and measures accountability in quantifiable terms. For example, the WS may hand over all their passwords to social media, install Life360 on their phones, check-in several times a day, and so on. All of these things are intended to provide some reassurance to the BS that the WS is indeed "where they said they would be and doing what they are supposed to be doing", but in all honesty, it's not all that helpful.
There isn't a BS alive won't immediately point out the obvious... which is that the WS can easily skirt any of these "accountability" efforts if they want to. They'll hand over all the social media accounts that you know of, but not the ones they still chat up their AP with. They'll leave their tracked phone at home and use a burner phone with the AP. Where there is a will, there is a way. Accountability by force is an illusion.
Accountability is what you do when other people aren't looking.
For me, accountability started with honesty. I made a pact with myself, for myself, to be as honest as possible about everything, every day. I also gave myself permission to fail and fuck up however. I did that, because I knew that if I put too much pressure on myself to be perfect from day one, I'd never make it. So every day, I did my best to just be honest about everything as often as I could, and just as importantly, I made a very focused effort to try and catch myself in my own lies, because it was so damn natural to me that I did it almost automatically. This extended to everything in my life. If I didn't get a work assignment done on time, I made an effort to just "own it" rather than making some lame excuse. If I ate the last cookie in the package, I said so. If I told a previous lie, I corrected it. When I screwed up, rather than get mad, I got observational. What was it I lied about? Why did I feel I needed to lie? How do I feel about myself knowing that I lied? What did it get me? How could it complicate things in the future? What would have been the outcome if I had told the truth?
Ultimately, telling the truth started to become its own reward. I noticed right away that, while it was sometimes hard to throw myself under the bus and admit that I had done something wrong, it got easier and easier to do so over time. Not only that, but the WEIGHT of the lies started to go away. In fact, I started to take pride and comfort in my own ability to tell the truth. I slept better at night. I never had remember what I "said" happened. Things I fucked up previously were rectified. And the most surprising thing of all (to me anyway) was that people actually APPRECIATED hearing me admit my mistakes. Taking ownership of my fuckups actually made people trust and respect me MORE, rather than seeing me as the consummate fuckup, which is what I always had pictured in my mind.
For me, the motivation to be more accountable came directly from how I felt about myself and the positive outcomes I experienced from being accountable in the first place. For my spouse, it became easier over time to believe and trust me as well, because she saw me take ownership of everything, even the things that didn't make me look good, or when I could just as easily have said nothing at all. With time and consistency, she saw the change in my behavior and thinking, she saw how I responded calmly to things that used to make me defensive, and that led to changes in my actions and in my ability to see her as well. One of the unadvertised advantages of being accountable is that it helps to remove the focus from ourselves, which in turn allows us to have empathy and concern for others.