I've not yet started on my receipt box. I'm still thinking about what I'll use as the box.
As far as impulse purchases go, I know exactly what my triggers are:
Clearanced items and "cheap" stuff that I can buy at Michaels or JoAnn's with a 40-50% off coupon. Over the past few years, I've spent more money on bunches of little things that I may or may not use than on more costly items I really want.
One of my 2012 craft goals is to make smart purchases, which means buying what I really want or need--even if it costs more than I think it should--and skipping all the "bargains" for items that don't make my heart skip a beat or are not necessary.