It isn't what you do 10% of the time that really makes a difference. It is the other 90% that really counts.
One or two diet sodas in a week isn't going to hurt you. A small piece of wedding cake or a cookie as you sit with that special someone won't kill your diet.
I try really hard to not have these things at home. I only attend 3-6 weddings a year, I can enjoy the cake. Birthdays are another thing. We have lots of birthdays...at least one a month. As long as we are not at my house, I can have 1/2 a piece with a little ice cream, at the end of the party. I leave before I can get another piece.
If we are at my house, it is harder. I try to have someone else do the serving onto the plates and then I serve the people. I save my piece for when I can't stand it any more. I do everything I can, pick up the garbage, go outside to play with the kids, sit and visit with other people. Then - and this is a big rule for me - I can't eat the last bite.
This works for me. Just sharing