Watch the Elbow
During the act of a high five, if one is having trouble, all he need do is "Watch the Elbow." If two people are going for a high five and both look at the others elbow, the chances of a "miss" decreses nearly 100%. It will work with less precision if only one person watches the elbow.
"Hey man, thanks for the great party, watch the elbow!" Proceed with a perfect high five