I’ve found that self-discipline is more of a habit for me.  
I’m impulsive and scattered.  I start one project and drift off into many others along the way.  Discipline for me came in very small steps.  I decided I was going to walk every morning for 30 minutes (before I did anything else) because if I said I would walk today I might not remember to walk until I was getting ready for bed.  Walking became a habit and then I began building on that habit.  Whatever was a must do followed walking.  If I only walk for 15 minutes one day I’m okay with that as long as I maintain the discipline of walking. 
Sometimes I got caught up in where I lacked in discipline and focused on how I didn’t achieve that particular disciplined activity.  Now I focus on what I did achieve instead of the mini “failures”.
Maybe you could make one day a week (or every other week) the ONLY DAY you will take clients to lunch.  This sets your mind up for a permanent event. The event may work with eating healthy too, if you set up one day a week to splurge on delicious less healthy stuff – for me it’s Totino’s pizza rolls 🙂