I just came back from a week's holiday which I was worried about for similar reasons to you. In fact I did not gain weight at all, which is unheard of for me on holiday. I didn't particularly try to eat clean and I drank alcohol most evenings but the following rules helped me:
1. Don't start the holiday eating before the holiday begins. Often with the mad rush getting ready and the feeling of "what's the point, I'm going to put this all back on on holiday" I tend to let my good eating slip the week before I go.
2. Eat a proper clean breakfast every day even if the rest of your meals are not great.
3. Only eat something that's not clean if it's really worth eating. So eat whatever you like but don't waste calories on stuff that doesn't taste that good or that you don't really really enjoy. (Obviously sometimes you have no choice.)
4. Don't stop eating your fruit and veg. I find I only have room for so much food and subconciously I tend to reduce my fruit and veg and fill up with other stuff.
5. Stop eating before you are really stuffed.
6. Don't let it spoil your holiday and don't feel guilty when you have to choose the less healthy options

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Ruth