For me, I'm probably not an FF as much as some people here, but I am overweight. Having said that, travelling seems not to do too much to my weight....
This is rather odd, since:
1. I eat almost everything offered on flights. Else, I'll stash it in my hand carry for later. Thankfully (or not thankfully?) most of the offerings in Y domestic are small.
2. I'm horrible at the QP. Food goes in lots and fast.
Some of the balancing acts:
1. I try to eat less carbs where I can. Not such a bad thing, because piling on the carbs after lunch tends to make me tired - not good at times (e.g. conferences). Also a good reason to give a miss to the token rock-hard roll on inflight meals...
2. Drink lots of water - this is difficult at times for me, because a handy source of water is not always at hand (and I'm not very keen on drinking straight from the tap - even in Australia), and at times I can easily drink anywhere from 500mL to 2L or more in one sitting.
3. Limit alcohol intake, particularly beer. For me, I don't usually drink as much as the typical red-blooded male of my age, so it's not all that bad. Even at the QP where the booze is free, I'd be lucky to knock back 2 drinks (and perhaps none if complementary bar is on the next flight).
4. My primary source of keeping active is walking. I have a pedometer with me and on a work day I'll try to walk at least 10000 steps per day. Average if I don't make 10k is usually 8k or so. When travelling, I try to walk as much as I can. Good way to discover a new location! Walking up and down new airports also makes for a good discovery journey.
Walking also serves as a good indicator of relative fitness. I have a rough idea of my endurance when I walk; if I find it harder to walk a certain distance under certain conditions, then I know something isn't right......
5. Changing small habits. For instance, I rarely have margarine or butter on bread. It is extremely rare when I add salt to something, irrespective of the taste. Drinking tea - especially green tea or Chinese tea - after a heavy meal is not a bad idea. Limiting coffee intake is also a good idea (especially instant coffee and/or decaf - these are particularly bad); while I like my coffee, at times I'll give it a miss and opt either for water or black tea.
So, while I'm not considerable to be deemed as in-shape (and no, round is not a shape!) - of which many here can pay witness to - I do have a fair idea on how to curb the weight gain, but to get back on topic, FF travel never makes that easy. Now I just have to work harder on the weight loss part.......