When I started my new eating plan, I promised myself for 3 months that I would eat totally on plan. No matter what happened in my life, I had to eat what was scheduled in my planner and not stray from it at all. Adding coffee, tea, etc. was ok as it didn't impact my calories but all food items were non-negotiable. And that means I can still see my friends for coffee, etc. I also have "safe" foods with me at all times in my purse, such as a protein bar or veggies, etc. There is NO excuse for me to go off plan.
6 months later, I am still on plan. I haven't eaten in a restaurant or at anyone's house since then, unless I ate my own prepared food that I had with me.
I know this doesn't seem possible AT ALL. But you know what? Only 1 day back into my old routines would totally wreck anything that I had accomplished. Eating a cookie or soup not on plan would automatically make me think I should just have Chinese food for dinner and start over tomorrow. And then tomorrow would never be back on plan, I know this in my heart (or should I say gut?).
So now, my plan is "just 3 more months on plan" to see how much I can lose for the spring weather to get here (and buy some new outfits!). Who knows, maybe then I'll see how far I can go for the summer months.
Once I got into my groove, it really became easy and a challenge to stay loyal to myself. Plus I got addicted to the scale going down every week.
Good luck with whatever you do!