One mistake I see often is using too much weight when doing dumbbell curls. Not to say this is always wrong. I do occasionally like overloading on curls using momentum to aid the weight up while fighting gravity on the way down. This is not the time to do that. I recommend using very light weight when doing these. You should begin with your arms slightly forward from your torso and pinned at your sides. As you curl the dumbbells up twist your pinky inwards toward your torso. With lightweight I am able to curl my pinky slightly more than comfortable at the very top creating a tense contraction in my bicep. If you do this with your arms lazily at your side and use momentum you can make the movement 90% easier. Focus on form here and go slow. I find doing both arms at the same time is actually more difficult. It forces your upper body to stay neutral with very little sway.