For fun, I wanted to do at least one instructable on science, because I was recently at a science competition.
Step 1: Calling Tails
When calling tails, make sure that the person who flipped the coin, catches it with the other hand. According to my experiments, this worked around 60% of the time.
Step 2: Calling Heads
When calling heads, make sure the person flipping the coin caught it in the same hand they flipped the coin with. According to my experiments, this worked 55% of the time.