...is not something I always manage. Turns out, it depends whether you have the card the right way round! Yes - cardstock is made with long fibres all going in the same direction, and if you fold with the fibres, you'll get a better fold; if you fold at right angles to the fibres, you're more likely to get cracking! You can tell which way the fibres go by gently bending the paper between your hands in both directions - the easier one to bend is the bend that goes in the direction of the fibres, and the one that will fold better.
Here's Katrina Werner explaining it very well:
You really do learn something new everyday!