Harps, sorry if I'm late to the party -- I don't know how I missed this thread!
I was wondering what sort of pre-planning you do to prepare for the weekend. Do you have your meals planned out ahead of time, or are you finding yourself in the drive-thru lane because you don't have anything to prepare at home? Are you out and about and eating out because you let yourself get really hungry, or are you staying in and eating because you're bored and the food is close to hand?
I think that if you really want to get a handle on it, you need to identify where your determination is slipping.
I am usually pretty good on the weekends, and I attribute it to the following things:
1) I plan out my weekend meals just like I do my weekday meals. I have, on occasion, prepared them ahead of time like I do the lunches I take to work with me. This takes away the most dangerous excuse (to me, at least) which is "I don't know WHAT I want to eat..." Weekend dinners are on the week-ahead dinner schedule already, so hubby and I both know what we're in for every night.
2) I make sure to eat something for breakfast, even if I get up late. I'll make it something small if I'm eating breakfast close to the time when I know I'm eating lunchtime.
3) If I'm out doing errands I make sure to consider where I'm going to be when it's time for me to eat. If I go too long I get cranky, and that's when I make the worst decisions. I'll make sure to make time to go somewhere that I know I can get something healthy, or have time to go back to the house for lunch.
4) I've finally figured out my trigger zones in the house. If I'm just having a lazy day and lounging around with the dogs and the hubby, I get hungry and want to snack if I'm in the family room next to the kitchen. If I'm upstairs watching TV in the bedroom I'm like a whole different person. If the weather is nice I'll go out into the backyard with a book and a blanket and the dogs and lounge around out there. (Eventually the dogs stop licking my face and go about their business rolling around in the grass, sunning themselves like lizards, or sniffing everything they can find to sniff.)
5) So if I want to be lazy I can -- I just do it in a different room/area. But if I want to get something done, I've realized that if I keep my hands busy I don't get hungry. Whenever I find myself drifting over to the fridge I'll go upstairs, pop in a video, and fold some laundry or clean out my dresser drawers. Or I'll stay downstairs and vacuum or organize magazines. Even if I just do something for 20 minutes I find myself forgetting about whatever snack I was craving before.
So anyway, this was SOOOO long! But maybe there was something in there that will help you. I firmly believe that you only need to get ONE good day under your belt, and after that every day get a little easier.

Sorry for being so long winded!!