The Flaw: Going too heavy.
The Fix: This may seem obvious, like the ultimate curling cliché, but it bears repeating. Too many guys go too heavy on barbell curls, with no shirt-stretching gains to show for it. Strict sets of light-moderate-weight curls can and will add size to your arms. Take the 45-pound plates off the bar and try some 10’s instead.
The Flaw: Letting the upper arms move freely.
The Fix: When curling, the forearms will be moving up and down, of course, but the upper arms should remain stationary — not moving forward or backward. Keep your elbows pinned to your sides without flaring.
The Flaw: Leaning back.
The Fix: A little body English is OK on the last rep or two of a tough set, but otherwise the torso should remain vertical. When you start leaning back, not only is tension taken off the target muscles (the biceps), but the lumbar spine also is being compressed in a potentially damaging way.