[header] How to run or walk outdoors in hot weather [title] Dress appropriately. [step] It's important to wear running clothing that is both comfortable and able to wick away moisture from your body. If you sweat a lot, avoid wearing cotton as it'll just soak up the moisture and leave you with heavier clothing, which isn't conducive to your best performance. 
While there are many proprietary names for specialized fabrics that wick away moisture, such as dri-fit, coolmax, polypro, etc. , they all fairly much perform the same way by wicking away moisture and drying fast.