Choose the right carrier
In my five years of parenting, I have tried out most baby carriers on the market (if you want to know more, look at baby carrier shopping tips), including during the hottest and the coldest days of the year. I personally prefer Mei tais and the Ergo baby carrier during the summer. These days, there are new cooler versions of the Ergo carrier on the market. I don't own one, but they look great. Another wonderful option is an authentic Indian sari made out of cotton. A sari can be used in the same way as a wrap, but you have to fold it in half lengthways first. My Indian friends swears it's the coolest way to wear a baby in hot weather, and it looks really pretty too!
Dress your baby right
It goes without saying, but I will say it anyway. Your baby carrier essentially serves as clothing as well. In the summer, tiny babies do great in onesies and a diaper. Later on, shorts and a thin t-shirt are great options. You may want to cut down on the number of layers you wear yourself as well. Summer babywearing can get a little hot and sweaty, but then again, the benefits outweigh the cons in my experience. One also gets sweaty in strollers, and they can't be used everywhere. Preparing for your holiday with a baby? Look at air travel with a small infant too!