A good rule of thumb is to dress your baby the way you will dress, but with great care to protect and keep cool exposed skin. Since a baby doesn't perspire effectively, they can become overheated far more quickly than an adult.
If you're going to be indoors, dress your infant in loose-fitting, lightweight garments, preferably made from a natural fiber like cotton, which absorbs perspiration better than synthetic fabrics.
For the outdoors, light-colored clothes that cover baby's skin as much as possible and still stay cool are a necessity. Some bonus items are wide-brimmed hats to shield that sweet face from the sun rays even on cloudy days, since harmful rays can penetrate the clouds.
If you are moving around outside with your baby packed in the stroller, do NOT cover your baby with a blanket on a hot day. This can cause overheating very quickly. Instead, try using cooling towels in their stroller. These are highly convenient, as you can wet them with a little bit of water and they provide instantly cold material.
Another suggestion is to purchase a small fan for your stroller. Be sure to secure it safely to allow your baby the most air circulation, without any safety hazard. Most of the baby safe fans will easily stop spinning with just the tap of your finger, but keeping it out of their reach is still advised.
Even when lightly dressed, never leave an infant in a hot room, direct sunlight, or a parked car. Even a few minutes could cause their temperature to spike, and in extreme cases, may prove life-threatening.