If you're cutting a short length of wood, usually the cork-back ruler will work well enough. But for longer cuts, even that tends to slide a bit.
Here's a TIP that you might want to try.
Put at few pieces of double-sided carpet tape along the back of your ruler...Then put the sticky side of the tape against a piece of your clothing so it isn't QUITE so sticky then apply the ruler to the wood and make your cut. Your ruler will NOT slip!