One habit most people fall into on barbell curls is using the exactly same grip every time. It makes, I guess, since the standing barbell curl is such a staple meat-and-potatoes biceps lift that getting creative with it doesn't seem necessary. But like I always say, variety is key, even with utterly foundational exercises like this. Feel free to keep using your standard curl grip with the hands just outside the legs in the down position, but also, rotate in both wide-grip and narrow-grip barbell curls to maximize biceps short head and long head development, respectively. With wide-grip curls, spread your hands just outside shoulder-width on the bar; on close-grip curls, pull the hands just inside shoulder-width so that they're touching your thighs in the down position. In the context of a workout, alternate your grip every set – ie, shoulder-width on the first set, wide-grip on the second set, close-grip on the third set. If the workout calls for more than three sets, feel free to go back to shoulder-width for remaining sets.