Jiuzhaigou is located in high altitude region, so you are not suggested to do any strenuous exercise, eat more vegetables and fruits while drink less in order to avoid altitude sickness. You are suggested to take some common medicines with you, a small oxygen bottle (which you can buy in Chengdu) is preferred; The old and frail and people with high blood pressure, coronary disease and heart disease are not recommended to travel to Jiuzhaigou without the permission of their doctors.
Note: The altitude of Huanglong is higher than Jiuzhaigou, so do pay attention to the possible altitude sickness in Huanglong, even if you are ok in Jiuzhaigou. Do not run or jump a lot during your tour. An oxygen bag in Huanglong is highly recommended. Plus a full breakfast may help you avoid the altitude sickness.