Personally, I'm sick of all this carving-of-mystic-sigils on people. Okay, I get that blood is all symbolic, and it's a sign that the ritual in question is evil and blah blah blah, plus it's an excuse to take Spike's shirt off (he's becoming like William Shatner in TOS; any excuse for toplessness happily capitalized on by writers). But it's all -- well, bloody. It's been overdone, and while I think Spike hurts pretty, I'm over it.
I think it would be much more fun to do your standard Mystic Sigils in chocolate body paint. Or even honey (which has the added benefit of being more liquid, thus allowing you to include the standard convention of "stay perfectly still"). Or watercolors or something. A nice Seurat-style set of Mystic Sigils would be tons of fun.