Lots of things! Ordered from least forward planning required, to most:
- Just lock yourself in the toilets for 10 minutes and wash your... smellier parts using your hands, sink water and soap dispenser soap. Yes, it's not exactly dignified, but no-one's watching (probably), and it's more dignified than causing your neighbour to put medieval plague doctor balm under their nose - and there's always plenty of paper towels available for drying yourself off. Bit of water and soap on your hands, push clothing far enough to reach your... various crevices, quick scrub, dry off with paper towels, bin the paper towels. If parents can change babies in those things you can give yourself a quick scrub
- Bring a spare t-shirt or equivalent, choose an appropriate point to change (e.g. between connecting flights or after sleeping). It's always a good idea to have some clothes in your hand luggage anyway, in case your checked bags go missing
- Bringing spare clean socks is also a good idea if you prefer to take your shoes off on long flights and/or if there's a lot of walking before one of your flights (e.g. it's an evening flight and you won't have time to change before heading to the airport)
- Bring a plastic carrier bag or dry bag so that after changing, you can wrap your worn clothes up while still locked in the toilet and not stink anyone our while stashing them in your bag
- Bring chewing gum and/or one of those travel toothbrush/tiny toothpaste sets
- Bring wet-wipes, or a flannel and/or a small travel/gym towel to make the self-cleaning process easier. Some airlines might even have wet wipes available for passengers who like to use them to wash their hands before or after the meal, which you could ask for then use for a quick freshen up in the loos
- Some stores sell "anti-odor" or "anti-microbial" clothing, which could be an option for a frequently-stinky frequent traveller - but it might just be expensive quackery