Sticking to a Budget

There was an interesting post over at Blueprint for Financial Prosperity, about ways to make yourself stick to your budget. Some of these are things I should try, some are things I sort of already do. But when I say I follow a budget, it's kind of a lie.
I do track my spending very carefully. I have budget lines for a wide variety of items, and not that much falls into the "Miscellaneous" catch-all. For 2005, my year-end difference between budgeted and actual expenses was only $13! Does being $13 under budget for a year mean that I am a super-thrifty disciplined saver? Sort of... but what I think it really means is that I am a creature of habit! As I wrote about in my Rule #10, the thing that really keeps me in okay financial shape is that I have gotten to a point in my life where my income is more than enough to pay for my habits, and my habits don't include wildly expensive activities.
What I would like to be better at is actually setting goals to save more, and really trying to reach them. A couple of days ago, for instance, I noticed that I had only about $100 left in my monthly food budget. My budget for the month is $650, which works out to around $21 a day. With 10 days left in the month, I'd have to spend only $10 a day to make my budgeted goal. So did I plan out a menu and go to the grocery store for 10 days worth of inexpensive breakfasts, lunches and dinners? No. I had worked late, and gone to the gym late, and was tired and didn't want to deal with anything, so I went to a Japanese restaurant and blew $44 on sushi.
Does that mean I will now try to make up for it by living off only $6 per day worth of food for the rest of the month? Nope. But it does mean I will try to do better next month. I'll be living in yet another different temporary space, in which I hope I'll be more comfortable cooking. We'll see.
What do you do when you see a shortfall in your budget? Do you buckle down and close the gap? Or do you just let it go and try to make up for it the next month?