Change Your Grip
One of the many problems with curls is that they neglect the brachioradialis, a forearm muscle that crosses the elbow joint. This muscle will add size to the lower half of the upper arm, and completes the look of big arms since the forearms will be larger to match the biceps. To zero in on their development, switch to a neutral hammer-grip when performing your curls, and you'll be glad you did. As a bonus, you can also use this grip for chin ups, and you'll be killing two birds with one stone.