Sit in such a way that you feel completely at ease. Relax every muscle in your body, including the muscles in your face. The best way to relax the muscles in your face is to smile gently as you breathe in and out. Don’t make a great effort, or struggle, or fight as you sit. Let go of everything. This prevents backache, shoulder-ache or headache. If you are able to find a cushion that fits your body well, you can sit for a long time without feeling tired. — Thich Nhat Hanh (from How to Live #BK558)
In this video, Thich Nhat Hanh describes how just sitting can be fun.