Never fire on the spot. Even in cases of violence or theft, 'suspend pending investigation', evaluate the matter and take appropriate steps.
Document. Keep a record of wrongdoings that have occurred, even in straightforward cases.
Review. Go over and supplement the personnel file so you are comfortable with what could be provided to a government agency or lawyer outside the company who requests it.
Get both sides. Give the employee an opportunity to respond to charges before a final decision on termination is announced. Keep it calm and professional.
Be fair. Examine previous discipline of similarly situated employees. Consistency is the best defence to the charge of discriminatory treatment.
Don't lie. Never backdate or falsify a personnel file. The reprimand should reflect the entire history of the employee's problem.