JudyFoodie and boulangere have given you good info. The most important factor in making chocolate curls is the temperature of the chocolate - it's got to be warm. If the chocolate is too cold, you'll end up with small bits and shards. You can either do as boulangere suggests and rub the chocolate (a technique I've never tired) or you can take both your courage and your chocolate in hand and run the chocolate over a stove burner. Keep the chocolate high about the heat source and move it back and forth over the heat until the chocolate is only slightly warm (a matter of a couple of seconds).
If you can find a grocery store that sells bulk chocolate in blocks of a pound or so, that will give you the best results for the large curls you are looking for. Set the block on a cutting board, put on a apron, and brace the block with your abdomen. rub its surface vigorously with the palm of one hand to warm it up. Then quickly position the blade of a sharp chef's knife perpendicular to the chocolate, and drag it across the surface a couple of times, gently brushing the curls off onto a plate. Repeat until you have as many curls as you want/need. Refrigerate them for 15 minutes or so to chill them down before dropping them dramatically on top of your cake.
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