Enroll in a free throw contest. This will not only help you practice, but it is a great way to practice under pressure. One of the most popular free throw contests is hosted by the Knights of Columbus. There’s also a lot of basketball contests at local rec centers. Do a quick search of it online to determine if one is located near you. A local rotary club might also put one together every year.
Two nationally run free throw contests are the Elks Hoop Shoot and KoC Free Throw Campionship. Plus at basketball camps there a lot of shooting competitions like a free throw contest.