Assuming you score first before folding, which helps, the other suggestion I can offer is to take the sheet of card and don't fold it, just bend slightly. Then rotate it through 90 degrees and do this again. It should bend more easily in one direction than the tother, and that is the direction the grain is running in. If you fold in this direction you should get better results than if you fold against the grain.
Scroll down this link a bit and you'll find more detail -
http://glossary.ippaper.com/default....ge/article/213
folding paper
