Build out style tiles that are aspects of a site / project to indicate the look and feel of how it would bee with out the vagaries of a moodboard and the nailing down of an actual mockup.

This is references a little in Shape Up where layouts of sites are soooooo vague they can be interpreted in a few ways, but the gist is there.

I think a combination of both these things is the way to go. Gives vague wireframes for layout and a bunch of aspects for the feel.