The folded tea towel

It's a classic and very simple: just slide a folded cloth under your board to prevent it from slipping.
You can increase the grip by moistening the cloth.
Jar rubbers

Even more effective are the 4 jar rubbers, 1 at each corner. Their material is both supple and highly adhesive, ideal for holding your board in place perfectly.
Keep these 4 rubbers in a drawer, close to your work surface, and take them out systematically at the same time as your board.
To sum up: whenever you're cutting or trimming on a board, it's essential to secure the board, for good working conditions. Folded cloth, or even better, jar rubber, are very effective for this.