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.
5DIII, 1DX, 8-15/4L, 16-35 f4L IS, 24-70/2.8L II, 70-200/2.8L IS II, 70-300/4-5.6L IS, 200-400/4L IS 1.4x, Zeiss 15/2.8, 17/4L TS-E, Zeiss 21/2.8, 24/3.5L TS-E II, Sigma 35/1.4 Art, Zeiss Otus 55/1.4, 85/1.2L II, 100/2.8L IS Macro, Zeiss 135/2, 600/4L IS II