Yeah, like Cookingmama said, put the used stuff on the top of the chimney. It tends to not leave a lot of space in between the pieces, cutting down the air-flow and makes starting the chimney take a lot longer. I just add it to the top or even just spread the chimney of new stuff over top of the old stuff on the grate.
Absolutely no reason to not reuse it under normal circumstances. If I accidentally spill juices on the coals or forget to put a drip tray in (yikes) then I don't reuse that stuff. It can make a nasty smell when it's burned.