If you choose to spank
If you do choose to spank, make it personal. Use an open hand, not a closed fist, belt, or willow switch. You should feel the impact. Realize that there is an emotional cost to what you do that will require a reinvestment in the relationship. Understand that there are likely to be long-term consequences for both the child and relationship. Are the consequences worth a moment of obedience? Understand that as a child grows older spanking has greatly diminishing returns. Spanking has no future as a discipline tool.