It sounds like you are thinking more of "gastroesophageal reflux" where an infant spits up some of the stomach contents after feedings. This is very common to some degree or another in many infants during their first year of life. As you suggest, it happens because the muscle area at the bottom of the swallowing tube (where it meets the top of the stomach) does not close as tightly early on. This area does tighten up on its own within 12 - 18 months. There is not any medicine, exercise, or other treatment that is routinely recommended to make it tighten up earlier.
Simple ideas like frequent burping, placing the baby in a semi-upright position during and after feedings, and making sure feedings are made calmly without bouncing may help minimize the spitting. Remember that what comes out of the mouth with gastroesophageal reflux is seemingly effortless and should be differentiated from other causes for spitting up or vomiting in a young infant. Regardless of the cause for spitting, it is always important to watch for dehydration and to monitor how well the baby is growing.