I'm a relatively new convert to GTD, and I'm still developing my system. I'm using a 3x5 Circa notebook from Levenger for my main collection device. But I find I'm losing track of other scraps of paper, receipts, and so on because they fall out of my notebook. To save some money, I designed a DIY slash pocket that I can put together quickly (having more than one helps me keep things organized. You can see a picture of the slash pocket here, and the template is here. It's designed to be punched and put into a Circa or rollabind notebook.