I'm doing a mural for my kid's ortho office. It's a very large wall, and I've learned a lot from it. I've done murals before but never one quite this large.
1. Use chalk, not pencil, to sketch on the wall.
2. Paint a small scale sample on paper (the same color as the wall) to work through contrast, brush strokes, shadows/depth, and any special issues you think your piece may entail.
3. Consider lighting - what you see during the day may not be the same at night.
4. Inspect the walls carefully for nubs and indentions; either hide them with your paint/design, or fix them before you start.
I'll probably think of more later, but I'm beat :)