I recently went to a workshop with Larissa Hall Carlson on yoga for summer, focusing on ayurvedic tips for staying cool. Ayurveda, the sister science to yoga, makes so much sense to me - it is all about balance. During the warm months, to balance out the pitta element that is associated with summer and heat, we should stimulate cooling aspects. Everything has an effect, including what we eat, do, see, and feel. In terms of yoga, balancing pitta might mean focusing on more moderate classes, rather than the most physically challenging vinyasa class. Avoiding forceful breathing (such as ujjayi), closing the eyes, and letting go of competition can also keep your yoga practice cooling during the summer months.
Drinking more water is always important when it is warm, but adding the cooling flavors of cucumber, mint, or lime to water can enhance the refreshing effects. Cool foods such as melon, coconut, and salads will be more balancing than warm or spicy foods. Even wearing cooling colors like blue, green, and white can soothe the eyes and give a feeling of coolness.
I also like to chill out with nadi shodhana, a cooling breath in which you cover one nostril at a time. For most people, it is very calming and refreshing, and it can help with insomnia as well! This video from Larissa shows how it is done. How do you stay cool in summer heat?