1) Avoid over indulging in sweets and cold foods especially when in combination (for example ice cream). If you do have ice cream have some hot tea or soup with it. Better yet have watermelon instead, it's cooling and the white part is amazing for improving kidney function. Coconut water is also a lovely cooling beverage and is full of electrolytes. Cucumbers are also effective at cooling you down.
2) Avoid over eating. Food stagnation, besides being uncomfortable, will also over tax your digestive system. This goes for alcohol as well.
3) Keep your tummy and low back covered, especially in the evening, in cooler weather, or if it's windy out.
In other words gorging on ice cream and beer and then drunkenly hang gliding in a bikini would be tempting fate;-).
If you do end up with stomach funk, be it food or flu related, Po Chai pills are a good first resort. There's not a whole lot in the way of contraindications for them so I feel comfortable posting their name without examining each of you first. If you want to know more about Po Chai pills such as child and adult dosages or summer ailments just let me know.