Heads also come in both aluminum and copper, the former being soft and more malleable, while the latter is somewhat stronger and more durable, but harder to paste. It's good to have a selection of both on your rack, and obey the following.
Custom shape the head.
Before you place a head, examine the placement and determine if you should shape the head to conform to the seam, pod, or hole. Clean it out. Probably the number-one reason head placements rip out is because climbers fail to properly clean the placement. Many climbers use a plastic blow tube to blast out debris. Wire in the back.