Hopefully you have people there with you who support you. But don't forget that we're here for you, too!
I suppose it would be helpful to know just where you "undo" what you've done over the weekend.
Is it lack of exercise? Snacking? Eating out?
There are different ways to combat each one of those. If you don't mind sharing a little more detail, maybe you'll get specific ideas (beyond the tips which Coach Nancy so awesomely posted below).