A lot of folks swear by preacher curls for good bicep isolation. There are two big reasons why I don’t like the preacher curl. First, because of the angle, you won’t get a full contraction. Second, for me at least, the weird angle puts unnecessary strain on my bones and joints. So my recommendation for great bicep isolation is the incline dumbbell curl.
Dumbbell curls are great for biceps to begin with. And you will notice how every douche and fuck boy who wanders into your gym will make a bee line over to the dumbbell rack and start doing some curls. But without proper isolation, you will be doing a lot of cheating with your front delts and failing to get the most out of your work.
Find an incline bench or an adjustable bench and set it to about a 45° angle. Grab your dumbbells and let your arms hang straight down, which will be slightly behind your body. Now rotate your palms about 15° out and slightly flex your wrist. Without this flex, you’ll be killing your wrist.
As usual, keep your elbows pinned to your sides and start to curl. A full contraction will bring the dumbbells just below your shoulders and slightly out. I’ll leave it up to you on whether you want to alternate or do them simultaneously. Alternating may allow you to do slightly more weight, but you will also try to tuck your shoulder and cheat the weight up. Going simultaneously keeps you from doing this.
On the way down, keep your wrists flexed. This will help build the lower part of your bicep. Enjoy the pump afterwards.