If you know your hair has constant fly-aways then a good tip would be to get a plastic comb and spray it with hairspray. Then, comb through your hair so you don't get any or very few fly-aways. However if you are going for an overall messy look, no need to do this.
Secure your hairstyle with bobby pins, preferable ones to match your hair colour.
If you don't want to use any bobby pins or hairspray then you can also go for either a more cute look by curling any loose hairs or straightening them for a I Don't Care look.
Carry around extra hair ties (for you Americans) or hair bands (for us British) to make sure that if you feel like your hair is loosening you can add more hair ties/bands to keep it in shape. You can also carry around bobby pins or hairspray.
If you have straightened, curled or used any other heating method then avoiding water and humidity is probably a good idea. If you have decided to style your hair for the day with heat, contact with humidity or water will restore your hair back to its natural look.
There are certain hair products that supposedly work based on the weather or something but it's not going to make your hairstyle last through the day alone. They just help maintain the style you are going for.
If you would like to give them a shot then why not try out the John Frieda Weather Works Frizz-Ease Weather-Proofing Protection Climate. It says that it is "proven to reduce the style-ruining effects of any weather condition". You can also get the shampoo and conditioner for regular use and protection.
That's all I came up with but comment down below any other advice, tips or tricks.