It takes a bit of work to get comfortable with this one.
When I was having trouble with it, some who know more than I do suggested barring the second fret with the index finger and bowing the ring finger over to the fourth fret on the G string.
Bm-1.jpg
Bm-2.jpg
The pinky just hangs out in mid-air, as does the finger that I use to compliment people on their driving skills.
Here is a short vid from Ukulele Mike with exercises to strengthen your barre technique:
http://www.youtube.com/watch?v=2CO7zryA668Good luck!
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