yup, not so easy. It's amazing to see people like Jake or Aldrine do so much with their ukes standing up without a strap. I find that I really can't squeeze hard enough with my right arm to completely free my left hand without the neck falling.
I've thought about giving in and just using a strap. After all James Hill uses one. But so far I guess I don't want to be dependent on it.
So with "pracitce, practice, practice" I've managed to find ways to transfer the job of supporting the uke from hand to hand depending on the music. After awhile it happens without thinking about it.
By the way, I have the same problem when sitting, just not as bad. When seated I try to squeeze the uke between my ribs and right leg. Coming from a classical guitar backround where your left hand is completely free, it has taken some time to get use to this. I've seen others rest their ukes on their left thigh with their left foot elevated classical guitar style, but I haven't found this very comfortable.