I didn't prepare mentally. For me, talking about mental preparation would be an excuse to procrastinate:
"Oh ... I don't have the brain space to think about that now ... I'll do it when I'm ready ... after I settle into the new job ... after my first deadline passes ... after the High Holidays ... etc."
If I waited until I was mentally prepared, I don't think I'd ever start.
What happened for me, this time, was I changed one thing and a bunch of other things started to click into place. I went back to weightlifting to impress my girlfriend, who had gently scolded me for not getting enough exercise.
Once I was hitting the gym, almost immediately I wanted to clean up my eating. I started off simply not being a pig - exercising portion control - but as the weight started coming off, I wanted to keep it coming off, and so I started counting calories more closely.
I guess the punchline for me is, no thinking - just doing. It's not about mental preparation - I know what to do, I just have to execute.