Lots of things! Ordered from zero forward planning required first, to some:

- **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](http://theoatmeal.com/comics/airplane_peekaboo)), and it's more dignified than [causing your neighbour to put medieval plague doctor balm under their nose](http://travel.stackexchange.com/a/90414/3145) - 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 and clean babies in those things, you can give yourself a quick scrub
- **Ask an attendant if they have disposable wet wipes** you can use, or "hand sanitizer wipes". Many if not most airlines do, and some even give them to everyone to clean their hands before or after eating. Even those that don't often have a supply available: they're useful for parents, germophobes and people who are ill or travel-sick, as well as self-conscious sweaty passengers. Then lock yourself in the lav and use them to freshen up. This is a favourite item for ["10 things you didn't know you could ask for on a plane"](http://www.independent.co.uk/travel/news-and-advice/10-things-you-should-ask-for-on-your-next-flight-a7085886.html) type list articles: 

  > If you forget to pack your own sanitizing wipes, ask a flight attendant since they usually have them on hand.

- **Bring a spare t-shirt or equivalent**, then choose an appropriate point during the journey to change (e.g. between connecting flights, after changing climates, or maybe after sleeping). It's always a good idea to have some clothes in your hand luggage anyway, in case your checked bags go missing
- **Bring spare clean socks** if you prefer to take your shoes off on long flights and there's a lot of walking before one of your flights (e.g. it's an evening flight and you can't change or shower before heading to the airport) 
- **Bring a plastic carrier bag or [dry bag][1]** so that after changing, you can wrap 
your worn clothes up while still locked in the toilet and not stink anyone out while stashing them in your bag
- **Bring chewing gum, and/or one of those travel toothbrush/tiny toothpaste sets**, often sold at airports, *sometimes* not unreasonably expensive. [For example](http://www.gapyeartravelstore.com/travel-toothbrush-kit.html):

  [![enter image description here][2]][2]

- **Bring your own pack of [wet-wipes](http://travel.stackexchange.com/questions/44043/are-wet-wipes-subject-to-the-tsas-3-1-1-liquids-rule)**, or a flannel and/or a small travel/gym towel to make the self-cleaning process easier and reduce any embarrassment of having to ask an attendant for one (though trust me, they won't bat an eyelid, they've see *so* much worse...)
- 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](http://www.rodalesorganiclife.com/wellbeing/why-anti-odor-clothes-stink)


  [1]: https://en.wikipedia.org/wiki/Dry_bag
  [2]: https://i.sstatic.net/qCI3bm.png