We've sailed on Celebrity about 6 or 7 times in the past 5 years, and it's now easier to avoid gaining much weight, if any, since we're familiar with the food. We know what we really want to have, and what isn't as good as it looks, so we're much more selective.
But I think my best advice is to only take a literal 'taste' of dishes at the buffet. I like to try a lot of variety, but often only need a bite or two to be satisfied. Or I'll know what to go back to get more of, if it's really worth it. Hubby and I share a lot, too; especially with desserts. Again, a bite is usually enough.
Alcohol is a bigger problem, since with the drink package, we do indulge more than at home. And some coffee drinks can be a lot of calories, as well as juices and sodas that I may be more inclined to get since they're 'free'. We're now getting more selective with beverages, too.
On the last couple of longer cruises (more than a week), we found that we 'hit a wall' at about day 9 or 10, where we're sick of most of the food and not as apt to overindulge.