I always have (at least) one more card than the camera has slots. That way, you keep the files on the card as a backup, until your HDD is also backed up (in at least two places, preferably). Now that I have a 1D X, I write RAW to both cards, and I've got two or more copies of each image from the moment of capture.
I have two 2x2 64GB 1000x CF card sets for the 1DX and 2x (1 64GB 1000x CF + 1 128GB SD) for the 5DIII. Only once have I filled the 64GB cards before downloading though, but better safe than sorry. I have seen some very good deals on memory cards lately.
1DXII, 5DIV, 5DSR, 8-15/4L, 11-24/4L, 16-35/2.8L III, 24-70/28L II, 70-200/2.8L IS II, 100-400/4.5-5.6L IS II, 200-400/4L IS 1.4x, 17/3.5L TS-E, 24/4L TS-E, 35/1.4L II, 100/2.8L IS Macro, 300/2.8L IS II, 600/4L IS II, Zeiss: 15/2.8, 35/1.4, 55/1.4 Otus, 85/1.4 Otus, Profoto B1