Oh, I know this one, and the people who are telling you just to get you bladder more full are giving you entirely the wrong advice, because you can end up in a lot of pain that way!
I've been using the She-P for several years now, and it still isn't easy, and sometimes I can't manage. Marci has some great tips -- here is mine. What I have to do is the exact opposite of what you think you would do. Pushing to force the urine out DOES NOT WORK. What's required is relaxation . . . and for me, it's drawing up the pelvic floor muscles, which feels almost like you're trying to suck the urine back INTO yourself. (If you do yoga or pilates, you are familiar with this -- the Pilates article that originally introduced me to the idea said to imagine you were trying to suck a pea up inside yourself.) Combining this movement with an effort to relax will eventually work -- a small amount of urine will pass, and then it will stop, because I'll lose the relaxation. But doing it several times, eventually a stream will start and continue. You actually don't have to EMPTY your bladder, as it turns out . . . just getting enough urine out to relieve the discomfort is enough.
I have also found that this gets better with acute practice. Every cave diving trip, I have to master it over again, but by the end of the trip, it's MUCH easier. (Although I have a mental block about peeing when I'm in front of anybody, which made my buddies' trick of putting me #2 on the team really cruel.

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When you get it down, you will find that frog kicking actually HELPS . . . but I had to start by stopping and holding onto something, just like Marci describes.
It's amazing how visceral the inhibition against peeing in your clothes is!