Only lately have I been getting a feel for "building the lather on my face," as a couple of respondents already described.
I first run the brush under the hot water until filled, then just work the tips of the bristles over the soap for a bit; once I see lather beginning to build in the cup, that's done. Now the brush hits my face and the lather still feels less than "all the way there." But I just keep doing circles on my cheeks and neck, and can soon feel the lather growing thicker and slicker on my face.
Even more important than the soap-economy factor is, that this keeps the brush on my face where it belongs, lifting and softening the hairs.
Somebody on these forums keeps separate cakes in individual Glad Ziplock tupperware cups. Whatever thread that was in, there were some soap-economy tips there as well.
Edit: The thread was Soap Storage. I've tried four times to insert the link here, though, and give up. :) Use the search function in the bars above.