Enforce the consequences that were promised. Don't take any excuse at this point. Ignore begging and pleading and promises to do it next time. Enforce the consequences and let the child experience the results of his choice. If you give in this time and allow the child to escape the consequences, don't ever expect cooperation.
Children learn quickly when they experience the results of their choices.
When children know what is expected and simply forget, remind them in kind, considerate ways. Nagging and pleading destroy a parent's power. Children have no respect for a parent who nags, pleads, and begs before eventually moving on to the yelling and screaming stage. Remember, the goal is to get children to do what needs to be done, not to engage in power struggles. Use as few words as possible for reminders.