Don't attack them or put them down. Explain the decision, but acknowledge their strengths and contributions. Be generous with severance pay. Keep severance consistent companywide. Explain the severance package in writing.
Nothing makes a departing employee more angry than guessing about money and benefits.
Don't escort anyone from the building with armed guards or make them pack their belongings in front of colleagues. In the weeks that follow the separation, answer follow-up questions promptly and thoroughly. Don't keep people hanging. Listen for fallout. If you see disturbed behavior, consult a clinical psychologist immediately.