One of my favorite composition techniques is to include one element in the frame that simply doesn't appear to belong:
Such as the one fan with the big green hair in a crowd of Rays' fans at Tropicana Field...
One in every crowd
Or, this one red impatien in a full frame of green leaves...
Lone red impatien
Or, this single red pepper on a pile of sweet yellow peppers...
Red on yellow peppers