Start by minimizing rope friction both at the anchor and against the rock. Ledges are nice for standing on and setting an anchor, but the friction of a rope running across a ledge can make the rap line hard to retrieve. If you're rapping from a ledge, set your anchor high to reduce friction. And to prevent the knot from catching on the edge, shift the ropes so the knot lies just below the ledge. Always, the first person down tests the pull of a rappel, and the last down makes sure the lines are free of twists and clear of cracks and other rope-snagging obstructions.
Long rappels beg for trouble, so if in doubt, split the rap into shorter ones.
If practical, use one rope doubled and you'll eliminate that pesky, jam-prone joining knot. Such stretchy ropes will absorb most of your pulling force before it reaches the anchor point, rendering futile even Herculean yarding. Watch your rope ends while rappelling, since the unequal stretch of the lines can cause them to creep dangerously. The bad news is that you're stuck, and not even cutting the rope will help you descend. The good news is that you can safely return to the anchor and try again.