There's some fab tips on here, thanks OP.
I keep a wash bag packed with duplicates / miniatures of everything and refill / replace as soon as I get back. This includes a pocket for bobbles, kirby grips, sewing kit, safety pins & san pro. I keep a face mask in to perk me up after flights too.
Temple spa do fab miniature sets for this purpose that are refillable
Second pair of straightners left in case (been caught out using before I leave and forgetting to chuck in 🤦🏻♀️)
Nude undies, pjs & small clutch bag for evenings left in case
Pinch all the shower caps from hotel rooms - great for keeping grubby shoe soles off your clean clothes
Download films / series / books on iPad so not stuck if you have time to kill
Small make up bag in handbag with liquids kept in one of the airport plastic bags ready to pull out. This has my headphones & medication as well as wipes & clean knickers in case of delay
I have a lovely big, grey cashmere wrap that has been a godsend for cold flights / meeting rooms so now goes everywhere with me
I plan all my outfits a few days before so can make sure all washed, ironed and ready to go - usually choose 1 blazer / jacket and work round it with tops / skirt / dress / trouser combinations
Gym kit has visited a lot of hotels with me but doesn't get worn as often as it should so I will sacrifice it if I need room for more clothes 
My own teabags and travel mug